title: Seasonal variations in TSH serum levels in athyreotic patients under L-thyroxine replacement monotherapy.
abstract: BACKGROUND
  Whether serum TSH undergoes seasonal fluctuations in euthyroid and hypothyroid residents of temperate climates is controversial.
  
  
  METHODS
  Monthly TSH and thyroid hormone levels were cross-sectionally analysed in a large cohort of euthyroid subjects (n=11 806) and L-thyroxine (L-T4)-treated athyreotic patients (n=3 934). Moreover, in a small group (n=119) of athyreotic patients treated with an unchanged dosage of L-T4 monotherapy, hormones were measured both in the coldest and in the hottest seasons of the same year (longitudinal study).
  
  
  RESULTS
  No seasonal hormone change was observed in the euthyroid subjects except for a small FT3 increase in winter (+2.9%, P<.001). In contrast, the L-T4-treated athyreotic patients had significantly higher serum TSH values in the cold season when the FT4 values were significantly lower. The differences were more notable in the longitudinal series (TSH, 0.80 vs. 0.20 mU/L and FT4, 16.3 vs. 17.8 pmol/L in December-March vs. June-September, respectively). In these patients also serum FT3 values significantly decreased in winter (in the longitudinal series, 3.80 in winter vs 4.07 pmol/L in summer). Regression analysis showed that in athyreotic subjects, a greater FT4 change is required to obtain a TSH change similar to that of euthyroid controls and that this effect is more pronounced in the summer.
  
  
  CONCLUSIONS
  Athyreotic patients undergoing L-T4 monotherapy have abnormal seasonal variations in TSH. These changes are secondary to the FT4 and FT3 serum decreases in winter, which occur in spite of the constant treatment. The underlying mechanisms are unclear, but in some cases, these changes may be clinically relevant.
authors.affiliation: Endocrinology, Department of Clinical and Experimental Medicine, Garibaldi-Nesima Medical Center, University of Catania, Catania, Italy.
authors.forename: Damiano
authors.identifier: http://orcid.org/0000-0002-4034-6378
authors.initials: D
authors.lastname: Gullo
authors.affiliation: Endocrinology, Department of Clinical and Experimental Medicine, Garibaldi-Nesima Medical Center, University of Catania, Catania, Italy.
authors.forename: Adele
authors.initials: A
authors.lastname: Latina
authors.affiliation: Endocrinology, Department of Clinical and Experimental Medicine, Garibaldi-Nesima Medical Center, University of Catania, Catania, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Frasca
authors.affiliation: Endocrinology, Department of Clinical and Experimental Medicine, Garibaldi-Nesima Medical Center, University of Catania, Catania, Italy.
authors.forename: Sebastiano
authors.initials: S
authors.lastname: Squatrito
authors.affiliation: Endocrine Unit, Magna Graecia University of Catanzaro, Catanzaro, Italy.
authors.forename: Antonino
authors.initials: A
authors.lastname: Belfiore
authors.affiliation: Endocrinology, Department of Clinical and Experimental Medicine, Garibaldi-Nesima Medical Center, University of Catania, Catania, Italy.|CNR, Institute of Biostructures and Bioimaging, Catania, Italy.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Vigneri
mesh_terms: D000328:Adult; D003409:Congenital Hypothyroidism / Q000097:blood / Q000188:drug therapy*; D003430:Cross-Sectional Studies; D005260:Female; D020249:Hormone Replacement Therapy / Q000379:methods*; D006801:Humans; D007558:Italy; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D012621:Seasons; D050033:Thyroid Dysgenesis / Q000097:blood / Q000188:drug therapy*; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000008:administration & dosage* / Q000097:blood; D014284:Triiodothyronine / Q000097:blood
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine
keywords: L-thyroxine; athyreotic; hypothyroidism; hypothyroidism treatment; seasonal TSH variations