Seasonal Variation of Serum Thyrotropin Concentration and Thyrotropin Response to Thyrotropin-Releasing Hormone in Patients with Primary Hypothyroidism on Constant Replacement Dosage of Thyroxine*

Abstract

Ten patients with primary hypothyroidism (aged 32--66 yr), replaced on constant daily doses of L-T4 (mean +/- SD, 1.90 +/- 0.22 micrograms/kg BW), were used to examine seasonal variations in serum levels of thyroid-related hormones for a period of 14 months. Basal and peak TSH concentrations after TRH (500 micrograms) were higher in winter than in summer. Summer values for basal TSH were all normal (normal range, less than 4.8 microU/ml), while winter values were supranormal in 5 of 10 patients. Summer values for peak TSH were subnormal or normal (normal range, 5.0--40.0 microU/ml), while winter values were supranormal in 3 patients, with the remaining values being normal [log basal TSH, 0.511 +/- 0.438 vs. 0.084 +/- 0.244 (P less than 0.05); log peak TSH, 1.394 +/- 0.410 vs. 1.017 +/- 0.423 (P less than 0.05)]. Serum resin T3 uptake, T4, free T4 index(FT4I), T3, free T3 index, and rT3 levels did not vary seasonally, although T4 and FT4I tended to fall in the winter. The summer and winter QKd interval (the interval from the onset of a QRS complex in the electrocardiogram to the appearance of the Korotkoff sound at diastolic pressure), basal metabolic rate, and serum cholesterol concentrations were all within the normal range. Basal and peak TSH after TRH were inversely correlated with serum T4 and FT4I levels. The basal TSH concentration was further inversely correlated with the seasonally altering ambient temperature. These results indicate that during the treatment of primary hypothyroidism with constant doses of T4, 1) serum TSH and its response to TRH show seasonal variation, 2) the hypersecretion TSH in the winter is related to small changes in serum T4 and FT4I levels, and 3) the seasonal variation in the serum TSH concentration may need to be taken into consideration when evaluating the adequacy of a T4 replacement dose.

The pituitary-thyroid axis in healthy men living under subarctic climatological conditions

“…As presented above, the substitution doses of thyroid hormones could be increased in winter (Konno & Morikawa 1982). This may be advantageous also in goitre patients (Hegedus et al 1987).…”

“…These findings are mostly in line with those obtained from hypothyroid patients in Japan and in healthy subjects during an Antarctic winter. Hypothyroid patients with constant T 4 replacement tended to have low serum total T 4 in winter, which points to the greater disposal of iodothyronines during cold seasons (Konno & Morikawa 1982). After a 42 week residence in the Antarctic winter, serum T 3 levels following oral administrations of T 3 were found to be lower before it, indicating that the plasma clearance rate of T 3 increased in the cold (Reed et al 1990b).…”

“…In healthy subjects serum total T 4 levels were similar during all seasons, but the serum thyrotrophin (TSH) response to TSH-releasing hormone (TRH) was greater in winter than in summer (Konno 1978, Harrop et al 1985. In hypothyroid subjects, lower serum T 4 levels and higher serum TSH levels and TSH responses to TRH were found in winter rather than in summer (Konno & Morikawa 1982). In another study, involving large populations, the serum TSH levels of young, middle-aged and old Italian subjects were at their highest in December in the middleand old-age classes (Simoni et al 1990), but no seasonal changes were seen in younger subjects (Pasquali et al 1984, Simoni et al 1990).…”

Thyroid hyperactivity with high thyroglobulin in serum despite sufficient iodine intake in chronic cold adaptation in an Arctic Inuit hunter population

“…The influence of cold exposure on circulating thyroid hormone concentrations has been studied using a variety of study set-ups (24,25,26,27,28,29,36,37,38,39,40). A slight decrease in thyroid hormone concentrations in serum (26,27) has been observed.…”

“…Human cold exposure increased thyroidal iodide turnover in an environmental chamber study (24) and plasma clearance rate of T 3 in Antarctic expeditioners (25). This was associated with a small decline in T 3 in serum in Antarctic visitors (26,27) and subarctic inhabitants (28), and was in keeping with an increased requirement for thyroid hormone during winter in hypothyroid individuals (29). None of the studies included data on iodine excretion, though this is a major environmental factor that influences thyroid activity, and Arctic residents remain to be investigated.…”