Comparative indexes of thyroid function in sheep

¹ Departments of Animal Husbandry and Physiology and Pharmacology, Michigan State University, East Lansing, Michigan

The effects of light and temperature on thyroid activity of sheep were assessed by four methods: thyroid secretion rate, zero time per cent uptake of I¹³¹, I¹³¹ output half-time, and measurement of the thyroid epithelial cell height. Increased ambient temperature inhibited thyroid activity as evaluated by all methods. Reduced temperatures consistently resulted in a depressed uptake of I¹³¹. I¹³¹ output half-time had no relationship with actual hormone secretion. Light alterations imposed the same trend on thyroid function when activity was assessed by secretion rate and cell height measurement. As evaluated by these two methods the lowest thyroid activity was observed at 12 hr of illumination per day and increased with both increasing and decreasing light beyond this 1:1 light-to-dark ratio. When the three isotopic measures of thyroid activity were correlated with the histological method the highest correlation was found between cell height and thyroid secretion.