Abstract

Background: The variations in results regarding plasma concentrations of thyroid hormone of mature amphibia are related to species and season as well as to other physiological and environmental factors such as sex, nutrition, photoperiod or temperature. Studies of seasonal changes in circulating thyroid hormone levels in amphibia have disclosed changing patterns. Plasma T₄ level was high in a period before the breeding season in toads and frogs. T₃ is more potent hormone than T₄ in amphibia. The deiodination activity in liver was low before metamorphosis but showed a quick, transient rise during metamorphosis. The deiodination activity in liver reduced during post metamorphic growth but owing to the growth of the organ the total activity did not decrease significantly. In mammals, administration of 6-n-propylthiouracil (PTU) blocks most of the T₄ to T₃ conversion in liver and other tissues and abolishes the effect of exogenously administered T₄ .In tadpoles, undergoing metamorphosis, conversion of T₄ to T₃ blocked by PTU.

Materials and Methods: Female toads were procured locally and acclimatized into laboratory conditions. The animals under experimental group were treated with PTU as per experimental protocol. The control groups were maintained and treated with vehicle. Both groups were sacrificed and blood was collected. The radioimmunoassay (RIA) was performed to record the data of serum T₄ and T₃.

Results: The results from our study showed significant differences in serum T₄ and T₃ concentration between control and PTU treated group. Results showed that both serum T₄and T₃concentrations in mature toad decreased after treatment with PTU. These findings were indicating the inhibitory action of PTU on the iodide metabolism particularly transformation of iodide into its ionic form, its entry into the follicular cells, iodination of tyrosil residues and subsequent deiodination of T₄ to T₃.

Conclusion: The similarities of the thyroid hormone action found in mature toad in the study with those of other mammalian and non-mammalian vertebrates supports evolutionary conservancy of the nature of thyroid hormone action.

Keywords: Thyroid hormone (T₄ and T₃), Deiodinase enzyme, Propylthiouracil (PTU), Radioimmunoassay (RIA).