Serum principal iodothyronines and the influence of cold exposure associated with shearing in the sheep

The effect of cold exposure caused by shearing on serum thyroid hormone (TH) concentrations in sheep kept at an ambient temperature of 8.5°C was studied. While the deep body temperature fell to the lowest level 4 h after shearing the concentration of triiodothyronine (T₃) increased to a peak value at that time. Thyroxine (T₄) and metabolically inactive reverse triiodothyronine (rT₃) levels reached their peak value after 24 h. The $\text{T}{}_3\text{T}{}_4$ ratio reached a maximum at about 4 h and $\text{rT}{}_3\text{T}{}_4$ and $\text{rT}{}_3\text{T}{}_3$ ratios rose to maximum values about 24 h after shearing. This sequence of events suggest a biphasic response to cold—an immediate secretion of TH from the thyroid gland, followed by adaptive alteration in T₃ and rT₃ generation in the extrathyroidal tissues.