• J. Clin. Endocrinol. Metab. · Aug 2013

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

    • Tae Hyuk Kim, Kyung Won Kim, Hwa Young Ahn, Hoon Sung Choi, Hojeong Won, Yunhee Choi, Sun Wook Cho, Jae Hoon Moon, Ka Hee Yi, Do Joon Park, Kyong Soo Park, Hak C Jang, Seong Yeon Kim, and Young Joo Park.
    • Department of Internal Medicine, Seoul National University College of Medicine, Seoul 110-744, Korea.
    • J. Clin. Endocrinol. Metab. 2013 Aug 1;98(8):3420-9.

    ContextThe widespread use of thyroid tests in asymptomatic individuals identifies many patients with transient subclinical hypothyroidism.ObjectiveOur 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 SubjectsThis 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 MeasuresAge-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.ResultsDuring 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 monthly distribution of follow-up TSH levels indicated a biphasic pattern, ie, an increase during the winter-spring season and a decrease during the summer-fall season, with a maximal TSH difference of 0.69 mIU/L in subclinical hypothyroid and 0.30 mIU/L in euthyroid subjects. Normalization of subclinical hypothyroidism was increased 1.4-fold in follow-up tests during the summer-fall follow-up, whereas subclinical hypothyroidism increased 1.4-fold in euthyroid subjects during the winter-spring follow-up.ConclusionsThe season in which thyroid testing was performed was independently related to the transition between subclinical hypothyroid and euthyroid status. Seasonal variations in TSH concentration should be considered before deciding on treatment of subclinical hypothyroidism, particularly in the areas with a wide annual temperature range.

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