Effects of Supplemental Iodine and Season on Thyroid Activity of White-Tailed Deer

Blood samples were taken at 6-8 week intervals from 18 captive, female white-tailed deer (Odocoileus virginianus borealis) over a 2-year period to examine circannual changes in serum thyroid hormones and the effects of 3 levels of dietary iodine (+0, +0.2, +0.7 ppm) added to a basal diet generally containing 0.26-0.29 ppm iodine. Iodine (I) supplementation of the diet did not significantly affect T4, FT4, body weight, reproductive performance, milk I concentration, serum T3 and T4 levels of offspring, or dry weight and I content of the thyroid gland. Total thyroxine (T4) levels were higher (P < 0.05) in early winter and spring than in late winter, summer, and fall, and showed a close relationship with cyclic changes in feed intake and body weight. Free T4 (FT4) levels were highly correlated (r = 0.74, P < 0.001) with T4. Total tri-iodothyronine (T3) did not follow a clear seasonal pattern. Serum T3 and T4 levels were higher (P < 0.05) in fawns than in adult does and in nonlactating does than in lactating does. These results indicated 0.26 ppm I in a diet consumed ad libitum was sufficient to meet requirements for growth and reproduction in white-tailed deer. J. WILDL. MANAGE. 47(1):45-58 Thyroid hormones, thyroxine (T4) and tri-iodothyronine (T3), affect a variety of physiological processes including lipid, carbohydrate, and nitrogen metabolism, calorigenesis, growth and development, nervous system function, and reproductive performance (Bernal and Refetoff 1977, Underwood 1977:280-284). Thyroid activity in white-tailed deer has been studied to better understand the ways in which deer respond to different nutritional and environmental conditions (Hoffman and Robinson 1966, Seal et al. 1972, Byrne et al. 1974, Bahnak 1978, Bubenik and Bubenik 1978, Seal et al. 1978a). It is well established that whitetailed deer undergo cyclic changes in food consumption, body weight, body composition, and metabolic rate (Silver et al. 1971, Holter et al. 1977, Moen 1978). The relationship between thyroid activity and deer physiology, including changes in metabolic rate, nutritional status, catabolic and anabolic states, age, and reproductive phenology is still not clear. Among the endocrine glands the thyroid is unique in that its function is dependent upon a specific trace element-iodine (I). Insufficient hormone production resulting from I deficiency can reduce surviv l and impair reproductive performance. The Great Lakes region is known to be a naturally I-deficient area (Eldridge 1924) with a history of endemic goiter in humans (Olin 1924, McClendon 1939) and domestic animals (McCollum 1957:391392) before I supplementation began in the 1920's. Little is known about the I content of natural vegetation in the Great Lakes region and to what extent I might be limiting for wild herbivores. This study examined thyroid activity and reproduc' Michigan Agricultural Experiment Station Journal Article 9986. Supported in part by the Michigan Department of Natural Resources through Federal Aid in Wildlife Restoration Project W-117-R. 2 Present address: Department of Zoology, Washington State University, Pullman, WA 99164-4220. J. Wildl. Manage. 47(1):1983 45 This content downloaded from 157.55. 39.221 on Mon, 03 Oct 2016 06:25:24 UTC All use subject to http://about.jstor. org/terms 46 IODINE AND THYROID IN DEER * Watkins et al. Table 1. Composition of basal diet used for iodine studies with white-tailed deer.