The activity of the thyroid glands in relation to genetic differences in wool production

Abstract

Twelve adult castrate male Merino sheep from each of two flocks, genetically different in their capacity for clean wool production, were allocated to a high (H: 0.81 kg digestible dry matter day-1; 26 g N day-1) or low dietary regime (L: 0.42 kg digestible dry matter day-1; 9.5 g N day-1). The sheep weighed 47 kg (s.e. =0.8) at the start of the experiment, with sheep from the two genetic groups having similar average liveweights. The dietary treatments were imposed for between 4 and 10 weeks before eight samples of venous blood were collected from each sheep during 24 h. Each sheep was then injected with thyroid stimulating hormone (TSH), (1.25 unit/sheep) before seven samples of blood were collected during the following 24 h. The plasma from these blood samples was assayed for total thyroxine (T4) and tri-iodothyronine (T3). There were no significant differences between the flocks in either T3 or T4 before or after the injection of TSH. We concluded that differences in the activity of the thyroid gland were not necessary for the establishment of genetic variation in the rate of wool growth. The circulating levels of both T4 and T3 were greater in the sheep receiving the H dietary regime (T4: 81.4 v. 55.8 nmol L-1; P<0.5 and T3: 1.8 v. 1.0 nmol L-1; P<0.05). These differences were maintained after the injection of TSH, which approximately doubled the pre-injection levels of T4 in plasma 9-10 h after the injection. The levels of T3 also increased after the injection, reaching a maximum which was 2.3 times the average pre-injection concentration after 4-5 h.