Influence of the thyroid gland on ovarian function in the aging rat

The effects of underfeeding and manipulation of the thyroid axis on ovarian function were determined in young and old rats. The depressant effect of reduced food intake on ovarian cycling in young females was potentiated by chemical thyroidectomy, while young anestrous, underfed rats cycled when their diet was supplemented with thyroid extract. These observations iindicate that cycling aberrations in underfed rats may occur secondarily to an altered thyroid state. To determine if thyroid state influences ovarian function in old animals, constant estrous (CE) rats were underfed or chemically thyroidectomized. All underfed rats eventually cycled, while the response to chemical thyroidectomy alone, though still effective, was less dramatic. Realimented CE animals eventually returned to a pattern of constant vaginal cornification. Underfeeding had no effect on ovarian function in old recurrently pseudopregnant females however, these rats responded to thyroid treatment with renewed cycling. 4-6 (YC) 10-12 (PEP) month old females entered a persistent vaginal estrous condition when fed low doses of thyroid extract with their ad libitum diet. When the thyroid supplemented diet was discontinued, YC females resumed regular cycling, whereas the vaginal smear in 40% of the PEP rats remained cornified. Cycling could be restored in YC-thyroid induced CE rats by electrochemical stimulation of the medial preoptic area. These date suggest that senile deviations from normal cycling in the aging reproductive system may be affected by alterations in the thyroid state.