Variations in Plasma Thyroxine (T 4 ) and Triiodothyronine (T 3 ) in Eastern Wild Turkeys

Abstract

A total of 185 plasma samples, from 3 different groups of eastern wild turkeys (Meleagris gallopavo silvestris), were analyzed for thyroxine (T4) and triiodothyronine (T3) by radioimmunoassay. Of this total, 118 were from 4 pen-reared birds sampled biweekly for 14 months, 43 were from trapped birds, and 25 were from gobblers killed during the spring hunting season. Variations in plasma T, level corresponded to changes in environmental temperature and to the molting process. However, variations in the plasma T3 level of the pen-reared birds could not be correlated with seasonal changes. Total plasma thyroid hormones (T4 + T3) varied in the same manner as T4 alone. There were significant differences (P 0.05) due to sex or age were found in plasma T1, T, or T, + T3. J. WILDL. MANAGE. 41(4):650-656 Thyroid hormone deficiency (hypothyroidism), such as that produced by iodine deficiency, thyroidectomy, or feeding or of an antithyroid agent, has been shown to decrease egg production in domestic chickens (Winchester 1939) and to cause cessation of spermatogenesis in roosters (Maqsood 1952). Growth and development are also retarded by hypothyroidism (Winchester and Davis 1952, Singh et al. 1968). A decrease in thyroid function in free ranging populations of eastern wild turkeys might affect their reproductive performance, and thus, result in a decrease in the actual number of turkeys in that population. This study presents the possibility of using plasma levels of thyroid hormones as an indicator of the physiological status of eastern wild turkey populations. Before investigations can be undertaken to determine the existence of thyroid hormone deficiency on populations of free ranging wild turkeys, the following questions need to be answered: (1) What are the ranges for normal plasma levels of thyroid hormones? (2) Do the levels of these hormones exhibit variations due to sex, age, and seasonal changes? (3) Do values differ between samples obtained from pen-reared, trapped, and killed wild turkeys? (4) Since there are 2 principal thyroid hormones, triiodothyronine (T3) and thyroxine (T4), should both or only 1 be measured? The authors wish to thank W. H. Hamrick, Wildlife Biologist, Game and Fish Division, Alabama Department of Conservation, and J. R. Gwaltney, research assistant, Auburn University, for their assistance in obtaining blood samples from trapped and killed specimens. This research was supported in part by funds from a special appropriation from the Alabama State Legislature to the Alabama Cooperative Wildlife Research Unit. MATERIALS AND METHODS