Thyroid and reproduction in amphibians and reptiles.

The relationship between the thyroid gland and reproduction in amphibians and reptiles has been studied for more than 100 years. Most studies suggest a positive involvement of thyroid hormones with some aspects of reproduction, but some studies support a negative role for thyroid hormones at certain life stages. Comprehensive studies of gene activation/suppression by thyroid hormones and their absence at various levels of the hypothalamo–pituitary–gonadal axis coupled with observations of adrenocorticoid activity, reproductive performance, and metabolic involvement are needed to understand this complex relationship. Highlights: Most amphibian and reptilian populations are declining in the face of climate changes, pollution, and habitat destruction. Although the hypothalamo–pituitary–thyroid axis often is positively associated with reproduction in amphibians and reptiles, negative interactions also occur at some life stages and in some species. Comprehensive studies involving thyroid and adrenal activity and gene activation/repression linked with reproductive events are needed [ABSTRACT FROM AUTHOR]