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The Journal of Clinical Endocrinology & Metabolism Vol. 90, No. 1 124-127
Copyright © 2005 by The Endocrine Society

Lean Body Mass Is a Major Determinant of Levothyroxine Dosage in the Treatment of Thyroid Diseases

Ferruccio Santini, Aldo Pinchera, Alessandro Marsili, Giovanni Ceccarini, Maria Grazia Castagna, Rocco Valeriano, Monica Giannetti, Donatella Taddei, Roberta Centoni, Giovanna Scartabelli, Teresa Rago, Claudia Mammoli, Rossella Elisei and Paolo Vitti

Department of Endocrinology, University of Pisa, 56124, Pisa, Italy

Address all correspondence and requests for reprints to: Ferruccio Santini, Department of Endocrinology, University of Pisa, 56124, Pisa, Italy. E-mail: fsantini{at}endoc.med.unipi.it.


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Discussion
 References
 
Total body weight is usually employed to calculate the amount of L-T4 to be administered in patients with thyroid diseases. The aim of this study was to evaluate the effect of body composition on L-T4 requirements. Body composition was assessed by dual energy x-ray absorptiometry in 75 patients on TSH-suppressive L-T4 therapy after conventional thyroid ablation for differentiated cancer. The mean daily dose of L-T4 was lower in normal-weight (127.5 ± 21.3 µg/d) vs. overweight (139.4 ± 24.5) and obese (151.3 ± 29.1) subjects. There was a much stronger association between the L-T4 dosage and lean body mass (P < 0.001, r = 0.667) compared with fat mass (P = 0.023, r = 0.26). Measurement of regional tissue composition showed peripheral lean mass as the best correlate with the dose of L-T4 (r = 0.679, P < 0.001) whereas no correlation was observed with peripheral fat mass. In conclusion, individual L-T4 requirements are dependent on lean body mass. Age- and gender-related differences in L-T4 needs reflect different proportions of lean mass over the total body weight. An estimate of lean mass may be helpful to shorten the time required to attain a stable dose of L-T4, particularly in subjects with high body mass index values that may be due either to increased muscular mass or to obesity.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Discussion
 References
 
LEVOTHYROXINE (L-T4) IS commonly employed to correct thyroid hormone deficiency from various causes (1, 2, 3, 4) or to reduce serum concentrations of TSH in the management of thyroid cancer (5, 6, 7) or nodular goiter (8, 9). Both excess and insufficient thyroid hormone produce adverse effects in various tissues (10, 11, 12, 13), and careful monitoring is advisable to establish the optimal dose of L-T4 (14). Beside clinical evaluation, the sensitive immunometric TSH assays currently available allow precise L-T4 dosage in most patients (15). Nevertheless, substantial interindividual variations of L-T4 requirements occur depending on the patient’s age, gender, and body size (1, 2, 3, 4, 16, 17, 18). The presence of residual functioning thyroidal tissue, concurrent nonthyroidal diseases (2, 3), pharmacological agents (19), or specific physiologic conditions, such as pregnancy (20), may also call for adjustment of the L-T4 daily dose. There is a general agreement in literature that ideal body weight should be considered to calculate the amount of L-T4 to be administered in each patients, and evidence has been provided (21) that lean body mass is a predictor of the daily requirements for thyroid hormone in the elderly. Yet, the influence of overweight and obesity on the management of L-T4 therapy has not been fully elucidated. This study was undertaken to evaluate the effect of body composition, as assessed by dual energy x-ray absorptiometry (DEXA), on L-T4 requirements in a group of thyroidectomized patients on TSH-suppressive L-T4 therapy.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Discussion
 References
 
Patients

Seventy-five patients (52 females, 23 males) turning to our Endocrine Unit were enrolled after total thyroidectomy and 131iodine remnant ablation for differentiated thyroid cancer. According to current criteria for the management of thyroid cancer (22), they were apparently free of disease. In particular, basal and recombinant human TSH-stimulated serum thyroglobulin and antithyroglobulin antibody were undetectable, indicating no functioning thyroid tissue at the time of the study. Patients had been on suppressive doses of L-T4 [serum TSH 0.005–0.3 mU/liter, free T4 (fT4) and free T3 (fT3) within their normal ranges] for more than 6 months, without interruptions or dose adjustments during this time period. They were not taking medications known to influence thyroid hormone adsorption or metabolism, and they did not have concurrent nonthyroidal diseases at the time of the study. L-T4 was taken in the fasting state. Patients were assigned to one of three subgroups based on their body mass index (BMI): normal weight (BMI 18.5–24.9 kg/m2); overweight (BMI 25–29.9) and obese (BMI ≥ 30). In each subgroup, the enrollment was consecutive until 25 patients/subgroup were recruited. This enrollment criterion was followed to ensure an even distribution of various anthropometric measures within our study group. The clinical characteristics of patients from the three subgroups are shown in Table 1Go. Informed consent was obtained from all patients.


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  		TABLE 1. Anthropometric measures, L-T4 daily dose, and serum hormonal concentrations in the three subgroups of patients (25 patients/subgroup)

 
Serum assays

Blood morning samples were obtained 24 h after the last ingestion of L-T4. Serum TSH was measured by the chemoluminescent method (Immulite 2000, DPC, Los Angeles, CA). Serum free T4 (fT4) and fT3 were measured by immunometric method (Vitros, Ortho-Clinical Diagnostic, Rochester, NY). Normal values in our laboratory are as follows: TSH, 0.4–3.4 mU/liter; fT4, 7–17 pg/ml (9.0–21.9 pmol/liter); and fT3, 2.7–5.7 pg/ml (4.15–8.75 pmol/liter). Serum thyroglobulin antibody was assayed by an immunoradiometric assay method (ICN Pharmaceuticals Inc., Asse Relegem, Belgium). Serum thyroglobulin was measured using a commercial chemoluminescent immunometric assay (DPC). Serum leptin was assayed by an RIA kit (Linco Research, St. Charles, MO).

Evaluation of body composition

Total and regional lean body mass and fat body mass were measured by DEXA (Hologic QDR 4500A, Hologic Inc. Walthan, MA). DEXA scans were analyzed with the manufacturer’s whole-body version (Hologic Inc.). Peripheral values of lean and fat mass were calculated by adding up values measured in superior and inferior limbs.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Discussion
 References
 
As shown in Table 1Go, our study group ranged over a broad interval of body weight (46–123 kg) and BMI (19.1–40.4 kg/m2). Mean age was significantly lower in normal-weight as compared with obese subjects, reflecting the lower prevalence of obesity in young vs. middle-aged adults. The mean total daily dose of L-T4 increased in overweight and obese subjects as compared with normal-weight subjects, but the dose per kilogram was significantly reduced going from lean toward obese subjects. There were no significant differences in serum fT4, fT3, and TSH concentrations among the three subgroups. Furthermore, there was no association between the total daily dose of L-T4 and serum fT4, fT3, or TSH concentrations, as assessed by linear regression in the whole group (data not shown).

The relationship between the total daily dose of L-T4 and body composition was analyzed by simple linear regression (Fig. 1Go). As expected, there was a significant positive correlation between the L-T4 dose and total body weight (P < 0.001, r = 0.611). The association was much stronger when the L-T4 dose was correlated with lean body mass (P < 0.001, r = 0.667) than with fat mass (P = 0.023, r = 0.26), both measured by DEXA. Confirming the poor role of fat mass in determining the L-T4 requirement, no association was observed between L-T4 daily dose and serum leptin concentration. The mean total daily dose of L-T4 was significantly higher in male than in female patients (160 ± 29 vs. 130 ± 20 µg/d, respectively; P < 0.001 as assessed by Student’s t test), but there was no difference when the daily dose of L-T4 was corrected for body weight (1.76 ± 0.10 vs. 1.88 ± 0.12 µg/kg·d, respectively). Thus, as evident from Fig. 1Go, the gender-related variations of L-T4 requirement appear dependent on differences in total body weight, with lean mass content having the highest impact.



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  		FIG. 1. Correlations between the daily dose of L-T4 and total body weight (upper panel), total lean mass (middle panel), or total fat mass (lower panel) in 75 thyroidectomized patients on L-T4 therapy at TSH-suppressive doses. Open circles, Males; filled circles, females.

 
Measurement of regional tissue composition showed peripheral lean mass as the best correlate with the total daily dose of L-T4 (r = 0.679, P < 0.001), whereas no association was found between L-T4 dose and peripheral fat mass (Fig. 2Go). A weaker correlation was found between the total daily dose of L-T4 and trunk lean (r = 0.484, P < 0.001) or fat (r = 0.243, P = 0.036) masses.



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  		FIG. 2. Correlations between the daily dose of L-T4 and peripheral lean mass (upper panel) or peripheral fat mass (lower panel) in 75 thyroidectomized patients on L-T4 therapy at TSH-suppressive doses. Open circles, males; filled circles, females.

 
A significant negative correlation was observed between patients’ age and the L-T4 daily dose corrected for body weight (r = 0.25, P = 0.03). This relationship disappeared when the L-T4 daily dose was corrected for lean body mass, indicating that the age-related reduction of L-T4 requirements is associated to a decrease in lean body mass.


   Discussion
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
References
 
A stable dose of L-T4 in treatment of thyroid diseases is achieved when the amount of hormone entering the bloodstream equals the proportion that is metabolized, allowing constant serum concentrations of fT4, fT3, and TSH. When administered in the fasting state, about 80% of L-T4 contained in modern tablets is absorbed (23). After ingestion, serum T4 values peak at 2–4 h and return toward basal levels after about 6 h. The serum half-life of T4 approximates 7 d in the euthyroid state, and its clearance depends mainly on deiodination and to a lesser extent on other pathways, such as sulfation and glucuronidation (24). The daily replacement dose of L-T4 to normalize serum TSH in adult hypothyroid subjects is, on average, 1.6 µg/kg. The amount of hormone to be administered must be increased when suppressed values of serum TSH are desired. In both cases, however, individual adjustments are usually required to attain the optimal daily dose. Although the total daily requirements of L-T4 are related to body mass, unexplained difference can occur among individuals for the same age and body size, even in the absence of functioning thyroid tissue. In this regard, the role of obesity in affecting L-T4 disposal and therapy requirements has never been defined.

To gain further insights into the influence that body composition may exert on L-T4 therapeutical needs, we have studied a group of hypothyroid patients who had been totally ablated for thyroid cancer and had been placed on L-T4 at TSH suppressive doses. The advantages of this selection are: no functioning residual thyroid tissue, narrow ranges of serum thyroid hormones and TSH on L-T4 therapy, and good compliance to drug prescription, which is usually obtained in neoplastic patients under a strict follow-up. Although L-T4 therapy was carefully carried out and monitored, using the smallest dose required to suppress TSH secretion, it is possible that the slight thyroid hormone excess, necessary to reduce serum TSH, may introduce a limitation. In this regard, however, current evidence indicates that body composition is not affected in patients taking TSH-suppressive doses of L-T4 as compared with healthy controls (25). Body composition was analyzed by DEXA that allows precise and reproducible measures of total as well as regional fat mass and lean mass (26, 27).

On one hand, the results of our study suggest that excessive fat accumulation, as indicated by increased BMI values, would imply higher doses of L-T4 to attain the same TSH reduction as in lean subjects (Table 1Go). On the other hand, after looking at body composition, lean body mass appears as the best correlate of L-T4 daily requirements, whereas fat mass has little or no effect. This observation implies that T4 disposal mainly occurs within the lean compartment. Leptin is specifically produced by adipocytes and serum leptin concentrations are proportional to the amount of total adipose tissue (28). The absence of correlation between serum leptin and the total L-T4 administered to our patients confirms that the adipose tissue has a minor impact on L-T4 needs. Lean body mass has been shown to be superior to other measures of body size as a predictor of dosage for many drugs (29). This may not be surprising considering that most metabolic processes occur within this body compartment. In case of thyroid hormone, type 3 inner-ring deiodination, converting T4 to inactive reverse T3, has been demonstrated in skin, which accounts for a large proportion of lean body mass in humans (30). Furthermore, type 2 outer-ring deiodinase enzyme, converting T4 to T3, has been detected in skeletal muscle, making this tissue an important site for T4 degradation (31). The relevance of skin and skeletal muscle in T4 degradation was strengthened by our finding that lean mass in extremities, as a predictor of L-T4 daily dose, is superior to whole-body or trunk lean masses that include the visceral component. However, the total lean mass also correlates with liver volume and liver blood flow, and a good relationship has been observed between lean body mass and drug clearance for several pharmacological agents that are metabolized predominantly by the liver (29). Because several pathways of T4 metabolism take their place in liver, including type 1 outer-ring deiodination, sulfation, and glucuroconjugation (24), it is presumable that the relationship between L-T4 dosage and lean body mass partially reflects the hepatic contribution to L-T4 clearance. Lean body mass has been found to be a better determinant of thyroid volume than body weight, both in normal-weight and obese subjects (32). This observation is in line with our findings; taken together, these results indicate that requirements for T4, either secreted by the thyroid or therapeutically administered, depend on lean body mass.

According to the role of lean mass in determining L-T4 disposal, the absolute value of L-T4 requirement was higher in males than in females, as a consequence of gender-related differences in body weight and body composition. Finally, confirming previous studies (21), the well-known reduction of L-T4 needs associated to advancing age appeared dependent on a relative decrease in lean body mass.

We conclude that individual requirements of L-T4 in subjects receiving hormonal therapy for thyroid diseases are greatly dependent on the respective lean body mass and that age- and gender-related differences in L-T4 needs reflect different proportions of lean mass over the total body weight. Although individual monitoring of clinical and hormonal parameters remains essential for proper adjustment of the L-T4 dosage, we believe that an estimate of lean body mass may be helpful to shorten the time required to attain a stable dose, particularly in subjects with high BMI values that may be due to increased muscular mass, such as in athletes, or to excessive accumulation of adipose tissue, such as in obese subjects.


   Footnotes
 
This work was supported by Ministero dell’Istruzione, dell’Università e della Ricerca Scientifica, programmi di ricerca cofinanziati 2002 "Obesity: Phenotype characterization and relationship with pathogenesis"; Centro di Eccellenza AmbiSEN "Effects of environmental chemical agents on the endocrine and nervous systems"; Ministero della Salute, programma speciale di sperimentazione 2000 "Strategia multidisciplinare per la prevenzione e la cura dell’obesità e dei disturbi del comportamento alimentare."

First Published Online October 13, 2004

Abbreviations: BMI, Body mass index; DEXA, dual energy x-ray absorptiometry; fT3, free T3; fT4, free T4.

Received July 6, 2004.

Accepted September 21, 2004.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 

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Lean body mass determines thyroxine replacement dose
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