SEASONAL-CHANGES IN LEVELS OF GROWTH-HORMONE, SOMATOMEDIN AND THYROXINE IN FREE-RANGING, SEMI-DOMESTICATED NORWEGIAN REINDEER [RANGIFER-TARANDUS-TARANDUS (L)]

1. 1. Semi-domesticated reindeer of both sexes from two adjacent herds (A and B) in the Lødingen district of Norway were studied. From Herd A jugular blood samples were collected in summer (1974), and from Herd B both in summer and in winter (1975). 2. 2. Serum levels of growth hormone, somatomedin and thyroxine were measured, and, for Herd B, conspicuous seasonal changes in the three growth promoting hormones were found, both for the calf and the adult groups. 3. 3. Levels of growth hormone were reduced in winter as compared to summer: 21.6 ± 1.6 ng/ml vs 31.9 ± 2.2 ng/ml (calves), and 20.3 ± 0.9 ng/ml vs 25.5 ± 1.5ng/ml (adults). The same was found for thyroxine: 144 ± 9 mmol/1 vs 249 ± 10 mmol/1 (calves), and 157 ± 5 nmol/1 vs 223 ±7 nmol/1 (adults). Levels of somatomedin, on other hand, were higher in winter than in summer: 1.11 ± 0.10 U/ml vs 0.67 ± 0.07 U/ml (calves), and 1.02 ± 0.09 U/ml vs 0.53 ± 0.04 U/ml (adults). 4. 4. The observed seasonal changes may reflect one aspect of the physiological adjustments involved in the adaptation of the reindeer to arctic living conditions. © 1978.