La fonction thyroidienne chez l'unau ( Choloepus hoffmanni Peters)

Abstract

It has been confirmed that the BMR of the sloth, Choloepus hoffmanni, is 34% below that of mammals of similar size and that its rectal temperature is around 34.4°C. The relative weight of the thyroid does not depart from normal and its histological aspects are those of an active gland. The blood cholesterol (1.72 g/L) is higher than in various herbivores. The uptake of 131I by the thyroid is slow. About day 7 after the injection, 10–32% of the injected dose are taken up by the gland. Urinary and fecal excretion of tagged iodide are delayed. Only 23–56% is excreted in 14 days. After prevention of recycling, the radioactivity of the thyroid is halved in 14–35 days. The iodoamino acids of the thyroglobulin are qualitatively and quantitatively similar to those in other mammals. The protein-bound iodine in the plasma is low: 1.67 μg%. The biological half-life of injected thyroxin is 9.7 days. The thyroxin secretion rate is very low: 0.18 μg/kg “Corrected” body weight/day. Some of these processes may possibly be related to a low TSH blood level. The metabolic response to injected thyroxin is normal. It is concluded that the thyroid of Choloepus functions at a low rate and that it may be responsible for both the low oxygen consumption and the low body temperature.