Abstract
1. 1. Thyrotropins and gonadotropins purified from representatives of each tetrapod class and subunits of the mammalian hormones were tested for their ability to stimulate acute in vivo thyroxine release in a teleost fish. 2. 2. All intact hormones showed significant, intrinsic thyrotropic activity in this fish; subunits were inactive. 3. 3. Gonadotropins and thyrotropins capable of stimulating the teleost thyroid are therefore found in all tetrapod classes. 4. 4. Independent evolution of teleost fish has apparently led to an unusual situation in which the thyroid receptors for glycoprotein hormones are unable to distinguish between these structurally similar molecules from heterologous species.
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