title: Effect of seasonal changes on the transition between subclinical hypothyroid and euthyroid status.
abstract: The widespread use of thyroid tests in asymptomatic individuals identifies many patients with transient subclinical hypothyroidism.
  
  
  
  Our objective was to determine the effect of seasonal change on serum TSH levels and the transition between subclinical hypothyroid and euthyroid status.
  
  
  
  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.
  
  
  
  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.
  
  
  
  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 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.
  
  
  
  The 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.
authors.affiliation: Department of Internal Medicine, Seoul National University College of Medicine, Seoul 110-744, Korea.
authors.forename: Tae Hyuk
authors.initials: TH
authors.lastname: Kim
authors.forename: Kyung Won
authors.initials: KW
authors.lastname: Kim
authors.forename: Hwa Young
authors.initials: HY
authors.lastname: Ahn
authors.forename: Hoon Sung
authors.initials: HS
authors.lastname: Choi
authors.forename: Hojeong
authors.initials: H
authors.lastname: Won
authors.forename: Yunhee
authors.initials: Y
authors.lastname: Choi
authors.forename: Sun Wook
authors.initials: SW
authors.lastname: Cho
authors.forename: Jae Hoon
authors.initials: JH
authors.lastname: Moon
authors.forename: Ka Hee
authors.initials: KH
authors.lastname: Yi
authors.forename: Do Joon
authors.initials: DJ
authors.lastname: Park
authors.forename: Kyong Soo
authors.initials: KS
authors.lastname: Park
authors.forename: Hak C
authors.initials: HC
authors.lastname: Jang
authors.forename: Seong Yeon
authors.initials: SY
authors.lastname: Kim
authors.forename: Young Joo
authors.initials: YJ
authors.lastname: Park
mesh_terms: D000328:Adult; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007037:Hypothyroidism / Q000209:etiology*; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D012621:Seasons; D012923:Social Class; D013959:Thyroid Diseases / Q000209:etiology*; D013972:Thyrotropin / Q000097:blood
chemical_list: D013972:Thyrotropin