Thyroid Activity in Teleosts

Abstract

THE available evidence suggests that a calorogenic effect of the thyroid is doubtful in Teleosts. There has been a failure to modify the oxygen consumption, and therefore the heat production, by increasing or decreasing the amount of thyroid hormone available. Furthermore, the gland, as demonstrated in Phoxinus lævis and Lebistes reticulatus, unlike that of the higher animals, shows a regression with low external temperatures, and an increase of activity with high. This is emphasized by treatment with thiourea, for immersion of the fish in 0.05 per cent thiourea (changed twice weekly) results in an increase in epithelial height and a decrease in the amount of colloid present. These changes are more rapid when the treatment is applied at a constant high temperature. In this instance the activity has been measured arbitrarily by the epithelial height. For example, in October 1951, the gland of Phoxinus lævis showed a maximal structural effect after ten days at 25° C., whereas at 3° C. there was only a slight rise after three weeks (see graph).