New insights into thyroid hormone function and modulation of reproduction in goldfish.

A number of studies have provided evidence for a link between thyroid hormones and physiological or pathophysiological conditions associated with reproduction. Most of the information available is based on clinical observations in human or research in mammals. There are also a number of studies in non-mammalian species, primarily investigating thyroid and reproductive endocrinology in isolation. The findings demonstrate that hyperthyroidism or hypothyroidism are associated with altered fertility due to changes in the levels and activities of hormones of the brain-pituitary-gonadal axis. There appears to be a consistent pattern based on a number of studies in mammalian and non-mammalian species, linking thyroid with reproduction. Results obtained in goldfish suggest that increased levels of thyroid hormones may reduce overall reproductive function. Since thyroid hormones influence metabolism and are known to stimulate growth in most species, it is likely that increased thyroid hormone levels may divert energy from reproduction and promote somatotropic functions. This is particularly important in oviparous species such as fish since energy investment in females during reproductive season is very significant, and increasing thyroid hormone levels after ovulation may be a contributing factor in promoting growth response. Thyroid hormones will likely work in concert with other hormones to influence reproduction in fish and other vertebrates.