Seasonal thyroid hormone levels of free-ranging white-tailed deer (Odocoileus virginianus) in Ontario

Abstract

Blood samples from killed and livetrapped deer of southern and central Ontario were analyzed for thyroxine (T4) and triiodothyronine (T3). Mean seasonal levels for six sex-age classes of deer ranged from 150 to 400 ng/dL (T3) and from 6.0 to 18.0 μg/dL (T4). Seasonal variation of T4 was less pronounced than that of T3. Both hormones decreased in the fall as compared with spring and summer, especially in yearling and older bucks. T3 values were also low in the winter but increased in spring and summer for most tested categories of deer. Differences in seasonal T3 and T4 concentrations were found between male and female deer older than 1 year, and among sex-age classes of the same sex. Regional differences in thyroid hormone levels suggested a latitudinal gradient, with higher values in southern than in central Ontario. Hormone of artificially fed deer were elevated when compared with animals that consumed only natural browse. Hormone levels were also higher in deer provided with food supplements of higher protein content. Extremely low winter T3 and T4 concentrations were found in malnourished deer. It is concluded that thyroid hormones in deer blood are influenced by current food intake and quality of nutrition. The potential for T3 and T4 as indicators of range quality is discussed.