Annual variations in serum thyroid-stimulating hormone and thyroid hormones and in their responses to thyrotrophin-releasing hormone in the reindeer

Abstract

Abstract The reindeer in its natural habitat is subject to great annual variations in ambient temperature, illumination and nutrition. To ascertain the effect of these environmental factors on thyroid function, serum thyroid-stimulating hormone (TSH), thyroxine (T4), tri-iodothyronine (T3) and reverse T3 (rT3) concentrations were measured four times a year (2 June, 8 October, 21 November, and 24 February) in 14 animals housed outdoors at latitude 69°10′N. They all showed statistically significant (P<0·05) seasonal changes. Serum TSH and T4 were highest in February (623 ± 30 ng/ml and 287 ± 19 nmol/l respectively). TSH was lowest in October (318 ± 47 ng/ml) and T4 in November (199 ± 19 nmol/l). The T3 concentration was highest in November (3·0 ± 0·3 nmol/l) and lowest in June (1·8 ± 0·2 nmol/l). In contrast, rT3 was highest in June (3·6 ± 1·2 nmol/l) and lowest in November (1·9 ± 0·6 nmol/l). Thus, there was an inverse relationship between T3 and rT3 (linear regression r= −0·406, P<0·01). TSH, T4, T3 and rT3 responses to exogenous thyrotrophin-releasing hormone (synthetic TRH; 500 μg i.m.) were determined in ten animals. The magnitude of their response to TRH was significantly (P<0·05) dependent on the time of year. When compared with the control level all the parameters rose significantly (P<0·05). The greatest rise in serum TSH occurred in October (219 ± 151% and the smallest in February (66 ± 53%). The corresponding figures for T4 were 56 ± 60% in June and 11 ± 18% in February, for T3 143 ± 45% in October and 73 ± 46% in June, and for rT3 25 ± 13% in October and 12 ± 10% in February. In conclusion, significant annual variations occur in the reindeer serum concentrations of TSH and thyroid hormones as well as in their responses to exogenous TRH. Environmental factors greatly affect the pituitary-thyroid axis in this mammalian species living far in the North. The observed variations probably reflect an adaptation of the species to the annual environmental cycle. Journal of Endocrinology (1994) 141, 527–533