Effect of thyroid hormone on initiation of persistent oestrus in the rat

When rats are exposed to continuous illumination persistent oestrus and polyfollicular ovaries develop. Thyroidectomy at 24 days of age (juvenile rats) prevents the onset of persistent oestrus and polyfollicular ovaries under exposure to constant light, and irregular ovulation continues to occur. Replacement with 1.75 microng L-thyroxine in these rats produces a prolonged oestrus. However, in adult persistent-oestrous rats (90 days after exposure to continous illumination) thyroidectomy does not interrupt persistent vaginal cornification. In rats receiving 100 microng of testosterone propionate subcutaneously at 5 days of age, persistent oestrus and polyfollicular ovaries develop. Thyroidectomy at 24 days of age (juvenile) prevents the onset of persistent oestrus and the development of polyfollicular ovaries, however, ovulation is not observed. Replacement treatment with 1.75 microng L-thyroxine in these rats produces a prolonged oestrus and polyfollicular ovaries. However, in adult persistent oestrus rats, thyroidectomy at 130 days does not interrup persistent vaginal cornification. From these facts, it may be inferred that circulation of a physiological level of thyroid hormone in juvenile rats is necessary for the development of oestrogen binding receptors in the hypothalamus. Therefore, a hypothyroid state during the juvenile stage interferes with the development and maturation of hypothalamic controlled pituitary ovarian function.