Is the infertility in hypothyroidism mainly due to ovarian or pituitary functional changes?

The objective of the present study was to examine whether hypothyroidism af fects the reproductive system of adult female rats by evaluating ovarian mo rphology, uterus weight and the changes in serum and pituitary concentratio ns of prolactin and gonadotropins. Three-month-old female rats were divided into three groups: control (N = 10), hypothyroid (N = 10), treated with 0. 05% 6-propyl-2-thiouracil (PTU) in drinking water for 60 days, and T-4-trea ted group (N = 10), receiving daily sc injections of L-thyroxine (0.8 mug/1 00 g body weight) during the last 10 days of the experiment. At the end of 50 days of hypothyroidism no hypothyroid animal showed a regular cycle, whi le 71% of controls as well as the T4-treated rats showed regular cycles. Co rpora lutea, growing follicles and mature Graafian follicles were found in all ovaries studied. The corpora lutea were smaller in both the hypothyroid and T-4-replaced rats. Graafian follicles were found in 72% of controls an d only in 34% of hypothyroid and 43% of T-4-treated animals. Serum LH. FSH, progesterone and estradiol concentrations did not differ among the three g roups. Serum prolactin concentration and the pituitary content of the three hormones studied were higher in the hypothyroid animals compared to contro l. T-4 treatment restored serum prolactin concentration to the level found in controls, but only partially normalized the pituitary content of gonadot ropins and prolactin. In conclusion, the morphological changes caused by hy pothyroidism can be a consequence of higher prolactin production that can b lock the secretion and action of gonadotropins, being the main cause of the changes observed.