Seasonal changes in T3, FT4, and cortisol in free-ranging Svalbard reindeer (Rangifer tarandus platyrhynchus).

Serum levels of 3,5,3'-triiodothyronine (T3), free thyroxine (FT4), and cortisol were determined for free-ranging Svalbard reindeer during winter (March), summer (June-July), and autumn (October). A total of 48 animals representing adult males, adult barren, pregnant, and lactating females, and calves were shot, and blood sampled from their hearts. T3 and FT4 were found to vary seasonally, the levels being lower in winter than in summer or autumn. Differences due to state of pregnancy, sex, or age were represented by low FT4 levels in lactating females (summer) and rutting males (autumn), and high T3 levels in young calves (summer). Serum cortisol levels were found to change seasonally, summer levels being higher than winter or autumn levels in all groups of animals. This finding suggests that glucocorticoids (cortisol) do not affect the deiodination from T4 to T3 in reindeer. The seasonal changes in T3 and FT4 in wild animals were similar to the changes in captive animals. It is therefore concluded that the changes in thyroidal hormone levels do not indicate changes in basal metabolic rate in this species.