NEW YORK (Reuters Health) Jun 28 - Serum thyroid-stimulating hormone (TSH) levels change with the season, rising in the winter-spring and falling in the summer-fall, according to a study from Korea.
"The season in which thyroid testing was performed was independently related to the transition between subclinical hypothyroid and euthyroid status," the study's lead authors Drs. Tae Hyuk Kim and Young Joo Park from Seoul National University College of Medicine, told Reuters Health by email.
"Subclinical hypothyroidism is frequently reversible. In cases with minimal TSH elevation, it is prudent to perform follow-up tests in the summer-fall season before deciding on treatment, particularly in the areas with a wide annual temperature range," they said.
The investigators estimated the effect of seasonal changes on serum TSH levels and the transition between subclinical hypothyroid and euthyroid status in a retrospective longitudinal study of 1,751 subclinical hypothyroid adults and 28,096 euthyroid adults who underwent serial thyroid function tests. They used linear mixed models to calculate average TSH levels by month adjusted for age.
According to a report online June 14 in the Journal of Clinical Endocrinology and Metabolism, over 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 -- 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 subjects and 0.30 mIU/L in euthyroid subjects.
According to the researchers, "The normalization of subclinical hypothyroidism increased 1.4-fold on follow-up tests performed during the summer-fall, whereas subclinical hypothyroidism increased 1.4 fold in euthyroid subjects during the winter-spring follow-up."
They say the modest effect of test season on TSH level is "particularly important because it may affect the TSH distribution of the whole population and result in periodic changes in the relative prevalence of subclinical hypothyroidism and euthyroidism determined by a fixed upper TSH reference limit."
The investigators think it's important to account for seasonal variation when comparing serial TSH results.
In cases with a high likelihood that TSH levels will normalize, such as young subjects with minimal TSH elevation (4.1- 6.0 mIU/L), the investigators say it's "prudent to perform follow-up tests in the summer-fall season before deciding on treatment.
It's also "noteworthy," they say, "that individuals who converted to euthyroidism were still prone to develop subclinical hypothyroidism again because their final TSH was higher (geometric mean 2.43) that of the original euthyroid population.
Drs. Kim and Park said their findings were not a complete surprise, noting that "prolonged Antarctic residence was shown to increase TSH by approximately 30%, which suggests that prolonged cold exposure could affect TSH levels in human adults."
Dr. Linda S. Werner, a diabetes, metabolism, and endocrinology specialist with Endocrine Associates, of Bridgeport, Connecticut, also wasn't very surprised by the seasonal variation in TSH. "This is an interesting observation that could have a physiologic basis; it makes sense that cold would stimulate TSH," said Dr. Werner, who wasn't involved in the study.
SOURCE: http://bit.ly/11NPC0o
J Clin Endocrinol Metab 2013.
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