Increased gonadotropin levels in goldfish do not result in alterations in circulating thyroid hormone levels

To determine whether increases in gonadotropin levels are capable of altering thyroid function in goldfish, plasma thyroid hormone levels were measured following induced changes in endogenous gonadotropin secretion and injection of carp gonadotropin. Radio-frequency lesions placed in the nucleus preopticus periventricularis or monosodium-L-glutamate-induced lesions of the posterior nucleus lateralis tuberis (NLT) of the hypothalamus were capable of stimulating significant increases in plasma gonadotropin levels, but were without effect on plasma triiodothyronine (T3) or thyroxine (T4) at time intervals ranging from 5 hr to 10 days. Likewise, injections of a superactive analog of gonadotropin-releasing hormone resulted in profound increases in gonadotropin levels without associated changes in thyroid hormones. No changes in the circulating levels of T4 or T3 were observed in response to injection of purified carp gonadotropins whereas injection of bovine thyrotropin or carp pituitary extracts stimulated significant increases in T4. Radiofrequency lesions of the pituitary stalk or of the anterior NLT also resulted in significant increases in circulating levels of T4, but not of T3, at 10 and 30 hr postlesion. These results demonstrate that direct acute stimulation of circulating thyroid hormone levels is not an intrinsic action of endogenous goldfish gonadotropin and that activation of the reproductive system, leading to ovulation in some cases, is without effect on blood total thyroid hormone levels. Additionally, these results confirm that hypothalamic inhibition of the pituitary-thyroid axis exists in this teleost fish and demonstrate that interruption of this inhibition results in a time-dependent, high-magnitude increase in circulating thyroxine levels.