This Article Abstract Full Text (PDF) Submit a related Letter to the Editor Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Riis, A. L. D. Articles by Møller, N. Search for Related Content PubMed PubMed Citation Articles by Riis, A. L. D. Articles by Møller, N. Related Collections Thyroid Female Endocrinology Metabolism

Increased Protein Turnover and Proteolysis Is an Early and Primary Feature of Short-Term Experimental Hyperthyroidism in Healthy Women

Medical Department M (Diabetes and Endocrinology) (A.L.D.R., J.O.L.J., J.F., J.W., N.M.), Aarhus University Hospital, and Institute of Experimental Clinical Research (P.I., J.F., N.M.), Aarhus University, 8000 Aarhus, Denmark

Address all correspondence and requests for reprints to: Anne Lene Riis, Medical Department M, Aarhus University Hospital, 8000 Aarhus C, Denmark. E-mail: anne.lene.riis{at}ki.au.dk.

	Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Context: Hyperthyroidism increases energy expenditure, glucose turnover, lipolysis, and protein breakdown.

Objective: Our objective was to test whether increased protein breakdown occurs independently of other catabolic effects in mild experimental hyperthyroidism.

Design: We conducted a single-blind, randomized, placebo-controlled, crossover study. Protein dynamics of the whole body and of the forearm muscles were measured by amino acid tracer dilution technique ([¹⁵N]phenylalanine and [²H₄]tyrosine). All subjects underwent a 3-h study in the basal state followed by a 3-h euglycemic clamp study.

Setting: The study took place at a university clinical research unit.

Participants: Eight healthy women (24–46 yr old) participated.

Intervention: Intervention included 6 d thyroid hormone (T₄ 50 µg and T₃ 0.67 µg/kg·d) or placebo administration.

Results: Thyroid hormone administration led to mild T₃ hyperthyroidism with more than a doubling of T₃ levels and suppression of TSH. Energy expenditure and body composition was unchanged. Glucose infusion rates, forearm glucose uptake, and levels of lipid intermediates were also alike. Basal whole-body phenylalanine flux and tyrosine flux (reflecting whole-body protein breakdown) were increased (P < 0.05) as were whole-body protein synthesis rate (P = 0.05). Basal forearm rate of appearance and disappearance for phenylalanine (reflecting muscle protein breakdown and synthesis) were similar.

Conclusions: Mild short-term experimental hyperthyroidism increases whole-body protein turnover and breakdown before any measurable changes in energy expenditure or glucose and fat metabolism, suggesting that amino acid and protein metabolism is an early and primary target for thyroid hormone action in humans.

	Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Hyperthyroidism is a catabolic state with increased energy expenditure, increased glucose turnover (1), increased lipolysis (2), and increased protein turnover (3). These metabolic effects are likely to be of clinical importance, because hyperthyroidism reduces exercise performanc, and increases risk of cardiovascular mortality (4) and bone loss (5).

The increase in protein turnover includes increased protein breakdown at the whole-body level and increased muscle protein breakdown (6, 7). In patients with hyperthyroidism, muscle protein loss and subsequent loss of muscle function causes significant muscle weakness (8, 9) from which patients recover only after several months of euthyroidism. It is unknown whether the effects of thyroid hormone on protein metabolism are secondary to overall catabolism with a subsequent need for protein substrates for oxidative phosphorylation or a direct primary effect of excess of thyroid hormones in itself.

Experimentally induced mild hyperthyroidism is a model of hyperthyroidism, which may be exploited to examine early, direct metabolic actions of thyroid hormone in humans. This avenue has been pursued by Lovejoy et al. (10, 11, 12), who observed increased nitrogen loss and increased whole-body leucine protein breakdown after 1–2 months low-dose T₃ administration. These studies did not, however, allow dissection of whether this is a primary effect, because energy expenditure and lipid and glucose turnover also increased probably due to the relatively long period of treatment.

A major effect of hyperthyroidism is stimulation of lipolysis in adipose tissue (2). Lipolysis may be estimated by measurements of changes in serum lipid intermediates, i.e. free fatty acids (FFA) and glycerol and whole-body palmitate flux. The development of a microdialysis technique has enabled in vivo assessment of changes in interstitial glycerol concentrations in sc adipose tissue. The concentration of glycerol is taken as an index of lipolysis, and increased regional lipolysis has been reported in overt hyperthyroidism (2), but the effects of mild hyperthyroidism remain to be elucidated.

The purpose of the present study was to test whether the effects of thyroid hormone on protein metabolism are early and primary or secondary to overall catabolism. We used short-term, 6 d, administration of modest T₃ and T₄ doses to accomplish mild hyperthyroidism in eight healthy women. We used infusion of phenylalanine and tyrosine tracers combined with catheterization across the forearm bed to quantify muscle protein breakdown and synthesis in the basal state and after stimulation with insulin as well as amino acids. The lipolytic response to thyroid hormone was assessed by microdialysis.

	Subjects and Methods

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Subjects

In a single-blind randomized design, eight healthy women (24–46 yr) were studied twice with an interval of 2–3 months, once with placebo pretreatment and once with 6 d pretreatment with thyroid hormones. T₃ was given in a daily dose of 0.67 µg/kg body weight, and the calculated total dose was divided in four daily doses to minimize fluctuations in serum concentration during the day. L-T₄ in a dose of 50 µg was administered once a day to meet the decrease in endogenous T₄ release from the thyroid gland due to the feedback-induced deficiency in TSH. The metabolic studies were undertaken on d 6 of the thyroid hormone pretreatment. Before investigations, study subjects received dietary instructions to maintain an energy- and protein-fixed diet. At both study days, the women were in the early follicular phase of their menstrual cycle. The forearm measurements could not be carried out in one subject because of problems with the catheterization. All participants gave their written informed consent after receiving oral and written information concerning the study according to the Declaration of Helsinki II. The Aarhus County Ethical Scientific Committee approved the study.

Materials

We used L-[¹⁵N]phenylalanine, L-[¹⁵N]tyrosine, and L-[²H₄]tyrosine from Cambridge Isotope Laboratories, Inc. (Andover, MA). The chemical, isotopic, and optical purity of the isotopes was tested before use. Solutions were prepared under sterile conditions and were shown to be free of bacteria and pyrogens before use. We used a commercially available amino acid solution, Glavamin (Fresenius Kabi, Uppsala, Sweden), 1 liter containing the following amino acids: 16 g L-alanine, 11.3 g L-arginine, 3.4 g aspartate 5.6 g glutamate, 30.27 g glycyl-glutamine (corresponding to 10.27 g glycine and 20 g glutamine), 3.45 g glycyl-tyrosine (corresponding to 0.94 g glycine and 2.28 g tyrosine), 6.8 g L-histidine, 5.6 g L-isoleucine, 7.9 g L-leucine, 9 g L-lysine (as acetate), 5.6 g L-methionine, 5.85 g L-phenylalanine, 6.8 g proline, 4.5 g L-serine, 5.6 g L-threonine, 1.9 g L-tryptophane, and 7.3 g L-valine in sterile water.

Methods and study design

The participants were admitted to the Clinical Research Center the evening before the day of examination. The investigations were carried out in the morning after an overnight fast (10–12 h) without any caffeine consumption or cigarette smoking; only ingestion of tap water was allowed, and the participants were placed in the supine position under thermoneutral conditions. Thyroid hormones and TSH were measured in the morning before administration of thyroid hormones. After regional catheterization (13), protein dynamics of the whole body and of the forearm muscles were measured by amino acid tracer dilution technique with infusions of [¹⁵N]phenylalanine and [²H₄]tyrosine and subsequent quantification of enrichment in plasma by gas chromatography mass spectrometry. Blood flow in the forearm was measured by plethysmography. All subjects underwent a 3-h study in the basal state followed by a 3-h hyperinsulinemic-euglycemic clamp study. Hyperinsulinemia was induced by a continuous iv infusion of regular human insulin 0.6 mU/kg·min (Actrapid; Novo Nordisk, Bagsvaerd, Denmark), and euglycemia was maintained by a variable infusion of 20% glucose adjusted to clamp the blood glucose concentration at 5 mmol/liter. Plasma glucose in arterialized blood was measured every 5–10 min in duplicate on an autoanalyzer (Beckman Instruments, Palo Alto, CA). To prevent unphysiological hypoaminoacidemia during insulin stimulation, we concomitantly infused an amino acid solution (Glavamin) at a constant rate of 1.056 ml/kg·h.

After priming the amino acid pool with bolus injections of [¹⁵N]phenylalanine (0.7 mg/kg), [¹⁵N]tyrosine (0.3 mg/h), and [²H₄]tyrosine (0.5 mg/kg), continuous infusions of [¹⁵N]phenylalanine (0.7 mg/kg/h) and [²H₄]tyrosine (0.5 mg/kg·h) were maintained for 6 h. Both in the postabsorptive state and in the clamp, blood samples were taken after 150 min with continuous infusions, when steady state was reached. The samples were drawn in triplicate over a period of 30 min. Enrichments of [¹⁵N]phenylalanine, [¹⁵N]tyrosine, and [²H₄]tyrosine were measured by mass spectrometry because their t-butyldimethylsilyl ether derivates under electron ionization conditions and concentrations of phenylalanine and tyrosine were measured using β-methylphenylalanine and -methyltyrosine, respectively, as internal standards (14). Whole blood concentrations of amino acids were determined by HPLC technique (Bio-Tek Kontron Instruments autosampler 465, System 265, and fluorescence detector SFM 25, Basel, Switzerland) with precolumn O-phthaldehyde derivatization. Blood samples were deproteinized with 10% 5-sulfosalicylic acid (15). Thyroid hormones (total T₃ and total T₄) and TSH were measured by immunofluorescent methods (Immulite; Diagnostic Products Corp., Los Angeles, CA). Free T₄ (fT₄) and fT₃ were measured by RIA (16, 17). We used a two-site immunoassay ELISA (18) to measure serum insulin. A double monoclonal immunofluorometric assay (Delfia, Wallac, Finland) was used to measure serum GH, whereas plasma glucagon (19) and serum C-peptide (Immunoclear, Stillwater, MN) were measured by RIAs. Serum FFA were determined by a colorimetric method employing a commercial kit (Wako Chemicals, Neuss, Germany). Serum levels of total IGF-I and free IGF-I were determined after acid ethanol extraction and ultrafiltration, respectively, as previously described (20). IGF-binding protein 1 (IGFBP-1) was determined by an in-house RIA (21) and IGFBP-3 by a commercial kit (from DSL Inc., Webster, TX). Ghrelin was determined by an in-house RIA as previously described (22).

Blood samples were deproteinized with perchloric acid for determination of glycerol, 3-hydroxybutyrate, and lactate by an automated fluorometric method (23). Indirect calorimetry (Deltatrac; Datex Instrumentarium Inc., Helsinki, Finland) was performed in both study periods to assess respiratory quotient (RQ) and to estimate total energy expenditure (EE). Lipid and glucose oxidation were estimated by correction for protein oxidation, as estimated by urinary excretion of urea (24). Urine urea excretion was determined by an indophenol method and serum urea by a commercial kit (COBASINTEGRA; Roche, Hvidovre, Denmark).

Anthropometric measurements and whole-body dual-energy x-ray absorptiometry (DEXA) scanning (Hologic QDR 1000/2000/W scanner) were performed to evaluate changes in body composition.

Phenylalanine kinetics were calculated as previously described (6).

Microdialysis fibers (CMA 60 microdialysis catheter; CMA, Stockholm, Sweden) with a molecular cutoff of 20 kDa were placed in abdominal and femoral sc tissue at the umbilical and midthigh level, respectively. After insertion, the catheters were perfused with physiological perfusion fluid [perfusion fluid T1 (CMA): 147 mM Na⁺, 4 mM K⁺, 2.3 mM Ca²⁺, and 156 mM Cl^– (pH 6); osmolality, 290 mosmol/kg], using a portable pump (CMA 106; CMA) at a flow rate of 0.3 µl/min, which is known to yield a recovery of almost 100% (25). After an hour of calibration, sampling was started at t = 120 min and continued until t = 360 min with 60-min intervals. The samples thus reflect the integrated level of interstitial metabolites during the preceding 60 min, and observed changes in interstitial glycerol concentrations reflect lipolysis (26, 27, 28), because negligible amounts of glycerol are reused in adipose tissue. The interstitial concentrations of metabolites measured by microdialysis represent mean values of samples collected during the last 2 h of each investigation period. The adipose tissue blood flow in the abdominal region was estimated simultaneously from the disappearance curve of injected ¹³³Xe as described previously (2).

Statistics

All the data were tested for normal distribution using SPSS for Windows 10.0 (SPSS Inc., Chicago, IL). Depending on this, either Student’s paired t test or Wilcoxon signed ranks test for related samples was employed for comparisons. Results are expressed as mean ± SEM. P values <0.05 were considered significant.

	Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Thyroid hormones and body composition (Table 1)

The thyroid hormone pretreatment resulted in mild T₃ hyperthyroidism raising total T₃ levels 150% and free T₃ levels 137% of placebo treatment values. TSH levels were suppressed, but T₄ remained constant. There was a slight increase in heart rate. Body weight and body composition did not change. Total EE and RQ were unchanged. None of the subjects had side effects of the thyroid hormone treatment, and the subjects were not able to tell which treatment they had been given.

Metabolites, hormones, energy expenditure, and insulin sensitivity (Table 2)

Treatment with thyroid hormones did not alter the fasting levels or the clamp levels of glucose, insulin, C-peptide, GH, glucagon, leptin, or ghrelin. Hyperthyroidism decreased total IGF-I, IGF-II, and IGFBP-1 levels, whereas free IGF-I and IGFBP-3 remained unaltered. The glucose infusion rates (M-values) necessary to maintain euglycemia remained alike. EE, RQ, and circulating lipid metabolites (FFA, glycerol, and 3-hydroxybutyrate) also remained unchanged.

Microdialysis (Table 3)

Microdialysis revealed no differences in interstitial glycerol concentrations in femoral or abdominal sc adipose tissue at any time point.

Whole-body amino acid kinetics (Table 4 and Figs. 1 and 2) After thyroid hormone pretreatment phenylalanine and tyrosine fluxes increased 10–20% in the postabsorptive state. Phenylalanine conversion to tyrosine was unchanged, whereas protein synthesis (phenylalanine disposal not accounted for by phenylalanine conversion to tyrosine) was increased. During the clamp, the same tendencies were observed, although not statistically significantly different.

View larger version (10K): [in this window] [in a new window]   FIG. 1. RaPhe, reflecting protein breakdown, in the postabsorptive state in eight healthy females treated with thyroid hormones or placebo, respectively.

View larger version (10K): [in this window] [in a new window]   FIG. 2. RaTyr, reflecting protein breakdown, in the postabsorptive state in eight healthy females treated with thyroid hormones or placebo, respectively.

Regional muscle amino acid kinetics (Table 4)

During the clamp, a similar net uptake of phenylalanine across the forearm was observed in both study groups. Insulin and amino acid stimulation of muscle protein synthesis (Rd_Phe) and inhibition of muscle protein breakdown (Ra_Phe) was also comparable in the two groups.

Amino acid levels Whole blood concentrations of amino acids are shown in Table 5. The basal total amino acid concentrations were elevated in mild hyperthyroidism, and in both study groups, the total amino acid concentrations increased 1.5-fold during the clamp. The elevation of amino acid concentrations was due to increased levels of branched-chain amino acids (leucine, isoleucine, and valine), whereas the gluconeogenic amino acids (alanine, serine, and glycine) were not significantly different. Serum urea and urinary urea excretion rates were comparable in the two settings (Table 2).

	Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

Many studies have shown that thyroid hormone increases energy expenditure in a dose-dependent manner (29) and that this is accompanied by loss of protein and muscle mass (6, 8, 9). In healthy male subjects treated with T₃ to produce a 3-fold elevation of total T₃ and 21% increase in basal metabolic rate, Gelfand et al. (30) observed increased energy expenditure, increased whole-body leucine flux, and increased nitrogen excretion. In addition, severe experimental hyperthyroidism (3- to 4-fold elevation of T₃) for 2 wk induced impaired exercise capacity, myopathy, and inhibition of oxidative and glycolytic enzyme activities (31). Tauveron et al. (3) also showed that 6 wk T₄ administration increased leucine turnover/breakdown, leucine oxidation, and leucine protein synthesis. Interestingly, this study also reported increased ability of insulin to decrease protein breakdown after experimental hyperthyroidism; these findings are in accordance with our observations of normal amino acid fluxes during the euglycemic clamp and underlines that the changes in protein metabolism are early and subtle. In experimentally induced mild hyperthyroidism, we observed no differences in amino acid kinetics during the euglycemic clamp with insulin, glucose, and amino acid infusions, and this is likely due to the more powerful anabolic effects of insulin coupled with high levels of amino acids. Finally, Lovejoy et al. (10, 11, 12) reported increased nitrogen loss and increased whole-body leucine protein breakdown after 1–2 months low-dose T₃ administration. All of these studies either reported increased energy expenditure or did not measure EE (which in all likelihood would have been increased) and therefore offer no answer to the question of whether increased protein turnover/breakdown is an independent and direct effect of thyroid hormone.

In our study, we found increased whole-body phenylalanine and tyrosine turnover and breakdown in the absence of increased energy expenditure. In this context, it should be underlined that this occurred in the presence of 5–10% increased circulating amino acid levels, which per se will tend to stimulate protein synthesis and inhibit protein breakdown (32, 33). More particularly, excess of branched amino acids, as observed in our study, has been shown to inhibit muscle protein breakdown (34, 35). It is all the more remarkable that whole-body protein breakdown remained 10–20% increased and that muscle protein breakdown across the forearm, if anything, tended to increase after thyroid hormone. Although not statistically significant, the magnitude of the increase in muscle protein breakdown was of the same order as that of whole-body proteolysis, suggesting that muscle proteolysis may contribute to the overall process. It should also be noted that any direct stimulation of muscle protein breakdown by thyroid hormone may be counteracted and concealed by the increase in circulating amino acids and that this increase may be viewed as a homeostatic mechanism preserving protein.

These observations were made in the absence of detectable alterations in resting energy expenditure and glucose and lipid metabolism. We employed state-of-the-art methodology to assess these metabolic indices, including indirect calorimetry, circulating concentrations of insulin, C-peptide, glucose, FFA, glycerol, and 3-hydroxybutyrate, arteriovenous catheterization to measure regional forearm exchange of metabolites, and microdialysis to measure interstitial concentrations of glycerol and regional lipolysis, together with a glucose clamp to measure insulin sensitivity. Although unlikely, it could be argued that turnover rates of glucose and FFA could have been altered. On the other hand, we found not only increased turnover rates for phenylalanine and tyrosine but also increased concentrations of total amino acids, branched-chain amino acids, and more particularly phenylalanine and tyrosine, clearly indicating altered amino acid metabolism. In contrast, no parameters of glucose and lipid metabolism were affected.

The mechanisms underlying the effects of thyroid hormone on protein metabolism are uncertain. In a recent study, Brennan et al. (7) reported that mixed muscle protein synthesis was increased in hyperthyroidism and that mRNA expression of myosin heavy chain was variable. Regarding protein breakdown, there is evidence that ubiquitin proteasome activity is decreased in muscle tissue from hypothyroid rats and that the activity is restored to normal levels after T₃ treatment (36).

Another interesting observation was that hyperthyroidism decreased IGF-I, IGF-II, and IGFBP-1 levels, whereas free IGF-I and IGFBP-3 remained unaltered. The effects of thyroid hormone on IGF-related parameters are controversial; in patients with longstanding Graves’ disease, some studies have reported unaltered levels and some have reported increased levels of IGF-I and -II (37). Our results clearly suggest that the direct effect of thyroid hormones is a decrease in IGF-I, IGF-II, and IGFBP-1 levels. It remains uncertain whether this is due to decreased rates of production or increased clearance; one possible mechanism could be a thyroid hormone-induced decrease in GH secretion (38). Although free IGF-I levels, maybe as a consequence of the suppression of IGFBP-1, were unaltered, the decreased total IGF-I levels could contribute to the increased protein breakdown, which we observed during hyperthyroidism. Decreased levels of ghrelin have previously been described in overt hyperthyroidism (39). If anything, we observed a weak trend toward lowering of ghrelin concentrations after thyroid hormone administration, indicating that suppression of ghrelin, like increases in energy expenditure and glucose and lipid turnover, is a later phenomenon occurring after the early increase in proteolysis.

It is important to underline that the model employed in the present study differs from others in that a modest dose of thyroid hormone was given for a short period of time. This allowed detection of early subtle metabolic changes. With more prolonged and pronounced exposure to thyroid hormone, the metabolic scenario becomes more complex due to changes in energy expenditure, substrate fluxes, hormone levels and sensitivities, and body composition. In our study, we accomplished comparable levels of all of these parameters. Thus, although not resembling overt clinical hyperthyroidism, our study may have clinical implications for patients with subclinical hyperthyroidism and emphasizes the importance of avoiding thyroid hormone overtreatment or performance-enhancing abuse of thyroid hormones, which will result in disturbances of protein metabolism. Such an effect is in accordance with the observation of decreased lean body mass after 6 wk low-dose T₃ treatment by Lovejoy et al. (9).

In summary, we have found that modest short-term thyroid administration increases whole-body phenylalanine and tyrosine turnover, implying both increased protein breakdown and synthesis. This occurs in the absence of detectable changes in energy expenditure, lipid and glucose metabolism, and major metabolic hormones and indicates that thyroid hormone affects protein metabolism primarily and directly and that this effect should be placed highly in the hierarchy of thyroid hormone action in man.

	Footnotes

 
Disclosure Summary: A.L.D.R., J.O.L.J., P.I., J.W., and N.M. have nothing to declare. J.F. has received consulting fees or lecture fees from Hoffmann-La Roche, Pfizer Germany. and Novo Nordisk Germany.

First Published Online July 15, 2008

Abbreviations: DEXA, Dual-energy x-ray absorptiometry; EE, total energy expenditure; FFA, free fatty acids; fT₄, free T₄; IGFBP, IGF-binding protein; Ra_Phe, phenylalanine rate of appearance; Rd_Phe, phenylalanine rate of disappearance; RQ, respiratory quotient.

Received March 10, 2008.

Accepted July 9, 2008.

	References

Top Abstract Introduction Subjects and Methods Results Discussion References

 

1. Møller N, Nielsen S, Nyholm B, Pørksen N, Alberti KG, Weeke J 1996 Glucose turnover, fuel oxidation and forearm substrate exchange in patients with thyrotoxicosis before and after medical treatment. Clin Endocrinol (Oxf) 44:453–459[CrossRef][Medline]
2. Riis ALD, Gravholt CH, Djurhuus CB, Nørrelund H, Jørgensen JOL, Weeke J, Møller N 2002 Elevated regional lipolysis in hyperthyroidism. J Clin Endocrinol Metab 87:4747–4753[Abstract/Free Full Text]
3. Tauveron I, Charrier S, Champredon C, Bonnet Y, Berry C, Bayle G, Prugnaud J, Obled C, Grizard J, Thieblot P 1995 Response of leucine metabolism to hyperinsulinemia under amino acid replacement in experimental hyperthyroidism. Am J Physiol 269(3 Pt 1):E499–E507
4. Biondi B, Palmieri EA, Lombardi G, Fazio S 2002 Effects of subclinical thyroid dysfunction on the heart. Ann Intern Med 137:904–914[Abstract/Free Full Text]
5. Faber J, Jensen IW, Petersen L, Nygaard B, Hegedus L, Siersbæk-Nielsen K 1998 Normalization of serum thyrotrophin by means of radioiodine treatment in subclinical hyperthyroidism: effect on bone loss in postmenopausal women. Clin Endocrinol (Oxf) 48:285–290[CrossRef][Medline]
6. Riis AL, Jørgensen JO, Gjedde S, Nørrelund H, Jurik AG, Nair KS, Ivarsen P, Weeke J, Møller N 2005 Whole body and forearm substrate metabolism in hyperthyroidism: evidence of increased basal muscle protein breakdown. Am J Physiol Endocrinol Metab 288:E1067–E1073
7. Brennan MD, Coenen-Schimke JM, Bigelow ML, Nair KS 2006 Changes in skeletal muscle protein metabolism and myosin heavy chain isoform messenger ribonucleic acid abundance after treatment of hyperthyroidism. J Clin Endocrinol Metab 91:4650–4656[Abstract/Free Full Text]
8. Nørrelund H, Hove KY, Brems-Dalgaard E, Jurik AG, Nielsen LP, Nielsen S, Jørgensen JO, Weeke J, Møller N 1999 Muscle mass and function in thyrotoxic patients before and during medical treatment. Clin Endocrinol (Oxf) 51:693–699[CrossRef][Medline]
9. Brennan MD, Powell C, Kaufman KR, Sun PC, Bahn RS, Nair KS 2006 The impact of overt and subclinical hyperthyroidism on skeletal muscle. Thyroid 16:375–380[CrossRef][Medline]
10. Lovejoy JC, Smith SR, Bray GA, Veldhuis JD, Rood JC, Tulley R 1997 Effects of experimentally induced mild hyperthyroidism on growth hormone and insulin secretion and sex steroid levels in healthy young men. Metabolism 46:1424–1428[CrossRef][Medline]
11. Lovejoy JC, Smith SR, Bray GA, DeLany JP, Rood JC, Gouvier D, Windhauser M, Ryan DH, Macchiavelli R, Tulley R 1997 A paradigm of experimentally induced mild hyperthyroidism: effects on nitrogen balance, body composition, and energy expenditure in healthy young men. J Clin Endocrinol Metab 82:765–770[Abstract/Free Full Text]
12. Lovejoy JC, Smith SR, Zachwieja JJ, Bray GA, Windhauser MM, Wickersham PJ, Veldhuis JD, Tulley R, de la Bretonne JA 1999 Low-dose T₃ improves the bed rest model of simulated weightlessness in men and women. Am J Physiol Endocrinol Metab 277:E370–E379
13. Møller N, Butler PC, Antsiferov MA, Alberti KG 1989 Effects of growth hormone on insulin sensitivity and forearm metabolism in normal man. Diabetologia 32:105–110[CrossRef][Medline]
14. Nair KS, Ford GC, Ekberg K, Fernqvist FE, Wahren J 1995 Protein dynamics in whole body and in splanchnic and leg tissues in type I diabetic patients. J Clin Invest 95:2926–2937[Medline]
15. Jones BN, Gilligan JP 1983 o-Phthaldialdehyde precolumn derivatization and reversed-phase high-performance liquid chromatography of polypeptide hydrolysates and physiological fluids. J Chromatogr 266:471–482[CrossRef][Medline]
16. Weeke J, Ørskov H 1975 Ultrasensitive radioimmunoassay for direct determination of free triiodothyronine concentration in serum. Scand J Clin Lab Invest 35:237–244[Medline]
17. Weeke J, Boye N, Ørskov H 1986 Ultrafiltration method for direct radioimmunoassay measurement of free thyroxine and free tri-iodothyronine in serum. Scand J Clin Lab Invest 46:381–389[Medline]
18. Andersen L, Dinesen B, Jørgensen PN, Poulsen F, Roder ME 1993 Enzyme immunoassay for intact human insulin in serum or plasma. Clin Chem 39:578–582[Abstract/Free Full Text]
19. Ørskov H, Thomsen HG, Yde H 1968 Wick chromatography for rapid and reliable immunoassay of insulin, glucagon and growth hormone. Nature 219:193–195
20. Frystyk J, Skjærbæk C, Dinesen B, Ørskov H 1994 Free insulin-like growth factors (IGF-I and IGF-II) in human serum. FEBS Lett 348:185–191[CrossRef][Medline]
21. Krassas GE, Pontikides N, Kaltsas T, Dumas A, Frystyk J, Chen JW, Flyvbjerg A 2003 Free and total insulin-like growth factor (IGF)-I, -II, and IGF binding protein-1, -2, and -3 serum levels in patients with active thyroid eye disease. J Clin Endocrinol Metab 88:132–135[Abstract/Free Full Text]
22. Espelund U, Hansen TK, Højlund K, Beck-Nielsen H, Clausen JT, Hansen BS, Ørskov H, Jørgensen JO, Frystyk J 2005 Fasting unmasks a strong inverse association between ghrelin and cortisol in serum: studies in obese and normal-weight subjects. J Clin Endocrinol Metab 90:741–746[Abstract/Free Full Text]
23. Lloyd B, Burrin J, Smythe P, Alberti KG 1978 Enzymic fluorometric continuous-flow assays for blood glucose, lactate, pyruvate, alanine, glycerol, and 3-hydroxybutyrate. Clin Chem 24:1724–1729[Abstract/Free Full Text]
24. Frayn KN 1983 Calculation of substrate oxidation rates in vivo from gaseous exchange. J Appl Physiol 55:628–634[Abstract/Free Full Text]
25. Moberg E, Hagstrom-Toft E, Arner P, Bolinder J 1997 Protracted glucose fall in subcutaneous adipose tissue and skeletal muscle compared with blood during insulin-induced hypoglycaemia. Diabetologia 40:1320–1326[CrossRef][Medline]
26. Jansson PA, Smith U, Lönnroth P 1990 Interstitial glycerol concentration measured by microdialysis in two subcutaneous regions in humans. Am J Physiol 258(6 Pt 1):E918–E922
27. Arner P, Bolinder J 1991 Microdialysis of adipose tissue. J Intern Med 230:381–386[Medline]
28. Hagstrom-Toft E, Enoksson S, Moberg E, Bolinder J, Arner P 1997 Absolute concentrations of glycerol and lactate in human skeletal muscle, adipose tissue, and blood. Am J Physiol 273(3 Pt 1):E584–E592
29. Bracco D, Morin O, Schutz Y, Liang H, Jequier E, Burger AG 1993 Comparison of the metabolic and endocrine effects of 3,5,3'-triiodothyroacetic acid and thyroxine. J Clin Endocrinol Metab 77:221–228[Abstract]
30. Gelfand RA, Hutchinson-Williams KA, Bonde AA, Castellino P, Sherwin RS 1987 Catabolic effects of thyroid hormone excess: the contribution of adrenergic activity to hypermetabolism and protein breakdown. Metabolism 36:562–569[CrossRef][Medline]
31. Martin WH, Spina RJ, Korte E, Yarasheski KE, Angelopoulos TJ, Nemeth PM, Saffitz JE 1991 Mechanisms of impaired exercise capacity in short duration experimental hyperthyroidism. J Clin Invest 88:2047–2053[Medline]
32. Volpi E, Mittendorfer B, Rasmussen BB, Wolfe RR 2000 The response of muscle protein anabolism to combined hyperaminoacidemia and glucose-induced hyperinsulinemia is impaired in the elderly. J Clin Endocrinol Metab 85:4481–4490[Abstract/Free Full Text]
33. Guillet C, Zangarelli A, Gachon P, Morio B, Giraudet C, Rousset P, Boirie Y 2004 Whole body protein breakdown is less inhibited by insulin, but still responsive to amino acid, in nondiabetic elderly subjects. J Clin Endocrinol Metab 89:6017–6024[Abstract/Free Full Text]
34. Louard RJ, Barrett EJ, Gelfand RA 1995 Overnight branched-chain amino acid infusion causes sustained suppression of muscle proteolysis. Metabolism 44:424–429[CrossRef][Medline]
35. Zanetti M, Barazzoni R, Kiwanuka E, Tessari P 1999 Effects of branched-chain-enriched amino acids and insulin on forearm leucine kinetics. Clin Sci (Lond) 97:437–448[Medline]
36. Solomon V, Baracos V, Sarraf P, Goldberg AL 1998 Rates of ubiquitin conjugation increase when muscles atrophy, largely through activation of the N-end rule pathway. Proc Natl Acad Sci USA 95:12602–12607[Abstract/Free Full Text]
37. Zimmermann-Belsing T, Juul A, Juul HJ, Feldt-Rasmussen U 2004 The insulin-like growth axis in patients with autoimmune thyrotoxicosis: effect of antithyroid drug treatment. Growth Horm IGF Res 14:235–244[CrossRef][Medline]
38. Ramos-Dias JC, Yateman M, Camacho-Hubner C, Grossman A, Lengyel AM 1995 Low circulating IGF-I levels in hyperthyroidism are associated with decreased GH response to GH-releasing hormone. Clin Endocrinol (Oxf) 43:583–589[CrossRef][Medline]
39. Riis AL, Hansen TK, Møller N, Weeke J, Jørgensen JO 2003 Hyperthyroidism is associated with suppressed circulating ghrelin levels. J Clin Endocrinol Metab 88:853–857[Abstract/Free Full Text]

This Article Abstract Full Text (PDF) Submit a related Letter to the Editor Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Riis, A. L. D. Articles by Møller, N. Search for Related Content PubMed PubMed Citation Articles by Riis, A. L. D. Articles by Møller, N. Related Collections Thyroid Female Endocrinology Metabolism