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Seasonal variations in TSH serum levels in athyreotic patients under L‐thyroxine replacement monotherapy
Author(s) -
Gullo Damiano,
Latina Adele,
Frasca Francesco,
Squatrito Sebastiano,
Belfiore Antonino,
Vigneri Riccardo
Publication year - 2017
Publication title -
clinical endocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.055
H-Index - 147
eISSN - 1365-2265
pISSN - 0300-0664
DOI - 10.1111/cen.13351
Subject(s) - euthyroid , medicine , endocrinology , hormone , thyroid
Summary 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 FT 3 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 FT 4 values were significantly lower. The differences were more notable in the longitudinal series ( TSH , 0.80 vs. 0.20 mU /L and FT 4, 16.3 vs. 17.8 pmol/L in December‐March vs. June‐September, respectively). In these patients also serum FT 3 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 FT 4 change is required to obtain a TSH change similar to that of euthyroid controls and that this effect is more pronounced in the summer. Conclusion Athyreotic patients undergoing L‐T4 monotherapy have abnormal seasonal variations in TSH . These changes are secondary to the FT 4 and FT 3 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.