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Raised serum TSH levels in patients with morbid obesity: is it enough to diagnose subclinical hypothyroidism?

Unit of Internal Medicine and Endocrinology, Fondazione Salvatore Maugeri I.R.C.C.S., ISPESL Laboratory for Endocrine Disruptors, Chair of Endocrinology, University of Pavia, Via S. Maugeri 10, I-27100 Pavia, Italy

(Correspondence should be addressed to L Chiovato; Email: lchiovato{at}fsm.it)

Objective: Morbid obesity (body mass index (BMI)40 kg/m²) is associated with thyroid function disturbances, with a high rate of subclinical hypothyroidism (SH) being the most consistently reported. We evaluated the circulating thyroid function parameters in morbid obese patients and related the results to the presence of circulating thyroid antibodies (Thyr-Ab).

Design and methods: Morbid obese patients were consecutively enrolled (n=350). Two control groups were used: control group (CG)1, healthy normo-weight subjects (n=50); CG2, normo-weight patients with SH (n=56) matched for TSH with the obese patients with SH. Serum levels of free triiodothyronine (FT₃), free thyroxine (FT₄), TSH, antithyroglobulin antibodies, and antithyroperoxidase antibodies were measured in all patients.

Results: i) Compared with CG1, obese patients having thyroid function parameters in the normal range and negative Thyr-Ab showed significantly higher serum TSH and lower free thyroid hormones levels, but a similar FT₄/FT₃ ratio; ii) SH was recorded in 13.7% obese patients; iii) compared with CG2, obese patients with untreated SH had a significantly lower rate of positive Thyr-Ab (32.1 vs 66.1%; P<0.005); iv) no gender prevalence was observed in SH obese patients with negative Thyr-Ab; and v) the comparison of the untreated SH patients (obese and normo-weight) with CG1 demonstrated that in SH obese subjects, unlike normo-weight SH patients, the FT₃ levels were significantly lower. This resulted in a normal FT₄/FT₃ ratio in SH obese patients.

Conclusion: Thyroid autoimmunity is not a major cause sustaining the high rate of SH in morbid obese patients. In these patients, the diagnosis of SH itself, as assessed by a raised TSH alone, appears questionable.

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