Effect of seasonal changes on the transition between subclinical hypothyroid and euthyroid status.

Context: The widespread use of thyroid tests in asymptomatic individuals identifies many patients with transient subclinical hypothyroidism. Objective: Our objective was to determine the effect of seasonal change on serum TSH levels and the transition between subclinical hypothyroid and euthyroid status. Design, Setting, and Subjects: This was a retrospective longitudinal study of 1751 subclinical hypothyroid and 28 096 euthyroid subjects aged over 18 years who underwent serial thyroid function tests at a health screening center between October 2003 and May 2011. Main Outcome Measures: Age-adjusted geometric mean values of the TSH level by month were calculated using linear mixed models. Adjusted odds ratios of test season and multiple baseline clinical factors were determined using generalized estimating equations. Results: During a median 36 months of follow-up, 57.9% of subclinical hypothyroid subjects reverted to euthyroidism, and 4.3% of euthyroid subjects developed subclinical hypothyroidism. The mo...