Some Effects of Thyroxine and Antithyroid Compounds on Tadpoles and Their Relation to Hormonal Control of Growth

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Abstract
Rana clamitans larvae were treated with the antithyroid compounds, 2-thiouracil and thiourea and also 3 dosage levels of thyroxine. These 2 types of treatments were administered both alone and simultaneously. The treatment with antithyroid compounds resulted in the characteristic increase in growth without affecting the rate of development of the stages examined. The 0.1 [mu]g./1. dosage of thyroxine had no effect. The 1.0 [mu]g./1. dosage was above the control level and caused hindlimb development in the treated series, but was not large enough to consistently reverse the effect of the antithyroid compounds. The 10.0 [mu]g./1. dosage produced an extreme hyperthyroid condition decrease in growth, profound metamorphic changes and uniformly reversed the effects of treatment with antithyroid compounds. It is concluded that 2-thiouracil and thiourea are effective as antithyroid compounds in anuran larvae and that the effects of treatment with these compounds are due to a lack of thyroid secretion. Thus, inasmuch as the effects of hypo- and hyperthyroidism are the opposite of those of the corresponding pituitary imbalances, it is suggested that the hormonal control of the normal growth process of anuran larvae consists of a balance between the antagonistic effects of the secretions of these glands.