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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