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Department of Endocrinology and Metabolism, University of Amsterdam, Academic Medical Center, 1105 AZ Amsterdam, The Netherlands
Address all correspondence and requests for reprints to: Leon J. S. Brokken, Department of Physiology, Institute of Biomedicine, University of Turku, Kiinamyllynkatu 10, FIN-20520 Turku, Finland. E-mail: Leon.Brokken{at}utu.fi.
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Abstract |
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Our findings suggest that TBII suppress TSH secretion independently of thyroid hormone levels, most likely by binding to the pituitary TSH receptor.
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Introduction |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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Recently we postulated a different mechanism to explain this phenomenon of sometimes long-lasting suppression of TSH values in otherwise euthyroid patients with treated Graves’ hyperthyroidism, involving a pituitary TSH-R (3, 4). We demonstrated that pituitary folliculo-stellate cells express the TSH-R and postulated that these cells could be involved in an ultra-short loop feedback control on TSH secretion (3). In this hypothesis, TSH secreted by the thyrotrophs binds to the TSH-R expressed on neighboring folliculo-stellate cells. These cells then secrete a messenger (e.g. a cytokine), which then suppresses TSH secretion by thyrotrophs in a paracrine way.
Such a TSH-R on the folliculo-stellate cells would also be recognizable by circulating TSH-R-stimulating Igs (TSI), because the pituitary resides outside the blood-brain barrier. These stimulating autoantibodies would then—just like TSH itself—down-regulate TSH secretion. This of course will go unnoticed during the hyperthyroid state but will become apparent when the patient is rendered euthyroid but still has circulating TSH-R autoantibodies.
This hypothesis was tested first in an animal model. Rats were chemically thyroidectomized using methimazole and kept euthyroid by adding T4 to their diet. They were then injected with either TSH-R autoantibody containing IgGs or control IgG. In agreement with our hypothesis, the TSI containing IgGs (and not the control IgGs) suppressed the TSH values without having an effect on serum levels of T4 or T3 (4).
Here we test this hypothesis in humans by following 45 patients with Graves’ hyperthyroidism treated with antithyroid drugs. When stable euthyroidism in terms of normal values for fT4 and T3 was reached, TSH values were related to thyroid hormone levels and TSH binding inhibitory Ig (TBII) titers.
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Patients and Methods |
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We performed a prospective clinical study in 45 consecutive patients with newly diagnosed Graves’ hyperthyroidism. This diagnosis was based on elevated levels of fT4 (>1.8 ng/dl [>23.0 pmol/liter]) and/or total T3 (>179 ng/dl [>2.75 nmol/liter]) in the presence of a decreased TSH (<0.4 mU/liter), positive TBII titer (>12 U/liter), and a diffuse uptake on a technetium scintigram. Excluded were patients with serious concomitant diseases, pregnancy, or on drugs known to influence the pituitary-thyroidal axis.
Forty-one patients were treated with 30 mg methimazole, and four with 400 mg propylthiouracil daily, to which was added L-T4 (109 ± 36 µg), aiming at normalizing fT4 (0.8–1.9 ng/dl [10.0–25.0 pmol/liter]) and total T3 (80–179 ng/dl [1.20–2.75 nmol/liter]) but avoiding elevated TSH values (>4.0 mU/liter).
When the patients were clinically and biochemically euthyroid for at least 3 months, their TBII levels were again determined and related to the levels of thyroid hormones and TSH.
Informed consent was obtained from all patients, and the study protocol was approved by the Medical Ethics Committee of the Academic Medical Center in Amsterdam.
Hormone assays
TSH plasma levels were measured with a highly sensitive chemiluminescent enzyme immunoassay that has a functional sensitivity of 0.01 mU/liter (Immulite Third Generation TSH kit, Diagnostic Products Corp., Los Angeles, CA). fT4 levels were determined with a solid phase time-resolved fluoroimmunoassay (Delfia, Wallac Oy, Turku, Finland). Total T3 plasma levels were determined by in-house RIA (5). To correct for effects of oral contraceptives on total T3 levels, the free T3 index (FT3I) was calculated as the product of total T3 and T3 resin uptake. The latter was determined with a T3 Uptake Kit (Ortho-Clinical Diagnostics, Amersham, Buckinghamshire, UK). TBII titers were measured by TSH-Rezeptor Antikörper assay (Brahms Diagnostica, Berlin, Germany).
Statistical analysis
Mann-Whitney U test was used to compare patients with negative TBII to patients with positive TBII with respect to fT4, FT3I, TSH, and several demographic parameters. We then calculated the correlation of TBII titers with thyroid hormone and TSH levels using nonparametric two-tailed Spearman’s Rho correlation. TSH values less than 0.01 mU/liter were substituted by 0.005 mU/liter in the statistical analysis.
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Results |
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. Mean age ± SD of the total group was 38 ± 12 yr; female/male ratio was 37/8; median (range) duration of the thyroid disease was 6 (1–120) months. After 6.7 ± 1.5 months of treatment with antithyroid drugs and L-T4, stable euthyroidism was restored in all patients. At that moment, 20 patients still had a positive TBII titer, and 25 had become TBII negative. fT4 and FT3I levels did not differ between patients with positive TBII and those with negative values. However, patients in the TBII-positive group had significantly lower serum TSH levels than those in the TBII-negative group [median (range) 0.09 (<0.01–4.30) mU/liter vs. 0.84 (0.01–4.20) mU/liter, respectively; P = 0.015 with Mann-Whitney U; Fig. 1, A–C].
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), whereas there was no correlation between TBII and fT4, FT3I, duration of the thyroid disease, or the L-T4 dose used by the patients. Similarly, no correlation was found between TSH levels and the latter parameters (Table 3).
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Discussion |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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). It thus seems that TBII are capable of suppressing TSH secretion, independently from thyroid hormone levels, most likely by binding to the TSH-R expressed on folliculo-stellate cells in the pituitary (3). In physiological circumstances, this pituitary TSH-R is probably involved in ultra-short loop negative feed back control on TSH secretion, but in Graves’ disease binding of TBII would also lead to suppression of TSH secretion, as suggested by our results. This explanation for the observed low levels of TSH in patients otherwise euthyroid during antithyroid treatment seems more plausible than to attribute it only to a delayed recovery of the pituitary-thyroid axis, for a number of reasons. First, patients with Graves’ hyperthyroidism do not in general have a long history of thyrotoxicosis. The delay between onset of symptoms and start of treatment in this study was only 4–6 months, and the duration of disease was not correlated to the observed TSH levels after 7 months of therapy. Secondly, in other cases of thyrotoxicosis, most notably T4 suppressive therapy in patients with papillary or follicular thyroid cancer, TSH becomes clearly elevated in a number of weeks after discontinuation of T4 treatment. Thirdly, our hypothesis explains why some patients with treated Graves’ hyperthyroidism attain normal TSH values in a couple of months and others do not. For, in approximately half of the patients TBII levels will become negative after a number of months, whereas in the other half of the patients they remain positive. The patients who remain positive for TBII have a higher relapse rate after discontinuation of antithyroid drug treatment, presumably because of the persistence of TBII. Other independent risk factors for a relapse of hyperthyroidism after a course of antithyroid drugs for Graves’ hyperthyroidism are persistent goiter, and the presence of low TSH values, even in the absence of detectable TBII. We therefore propose that the persistence of low TSH values can be seen as evidence for still circulating TSH-R-stimulating autoantibodies.
Our results also support the functional relevance of the pituitary TSH-R. We and others (3, 6) found the TSH-R expressed on folliculo-stellate cells, which have always been thought to be involved in the paracrine regulation of pituitary hormone secretion (7, 8). The presence of the TSH-R on these cells makes such a role more plausible, and we suggest that they might also be involved in the pulsatility of TSH secretion. Such an ultra-short loop control is probably not limited to TSH secretion, because other receptors for pituitary hormones, like the GH receptor and the prolactin receptor, have also been found in the pituitary (9, 10, 11). In fact, evidence is accumulating that both GH and prolactin can down-regulate their own secretion (12, 13).
We therefore think, that the postulated ultra-short loop feedback is operative in the secretion of all pituitary hormones. However, because of the specific characteristics of Graves’ disease (TBII), this will only have clinical consequences in this TSH-R-mediated disease. It should be noted that such an ultra-short loop feedback would only serve for fine-regulation of TSH secretion. In other words, if the patient truly becomes hypothyroid, this will override the fine regulation through the pituitary TSH-R, and TSH values may nonetheless become elevated.
In conclusion, this study shows that elevated TBII levels are associated with continued suppression of TSH secretion in patients with Graves’ hyperthyroidism rendered euthyroid, most likely via interaction with a pituitary TSH-R.
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Acknowledgments |
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Footnotes |
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Received March 11, 2003.
Accepted May 22, 2003.
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  G. Gelwane, N. de Roux, D. Chevenne, J. C. Carel, and J. Leger Pituitary-Thyroid Feedback in a Patient with a Sporadic Activating Thyrotropin (TSH) Receptor Mutation: Implication That Thyroid-Secreted Factors Other Than Thyroid Hormones Contribute to Serum TSH Levels J. Clin. Endocrinol. Metab., August 1, 2009; 94(8): 2787 - 2791. [Abstract] [Full Text] [PDF]
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