The effect of thyroid disorders and nigellar (Nigella sativa oil) medication on the regulation of the estrous (ovarian) cycle and fertility hormones of the rat was investigated. The chemical induction of thyroid disorders reduced the growth rate of female rats, and the oral treatment with nigellar ameliorated such reduction. The ameliorating effect was found to be more potent in hyperthyroidism, and it may depend upon its anabolic potency of fats and the accumulation of them inside the body and around internal organs. Thyroid disorders reduced the total number of the estrous cycles, while treatments with nigellar could not restore the normal number. Thyroid disorders induced disturbances in the follicular phase and in the luteal one as well. Nigellar medication could only ameliorate these disturbances in the follicular phase in hyper- and hypo-thyroidism, and could not improve luteal disturbances in both cases. Thyroid disorders also inhibited pituitary gonadotropins (FSH & LH) secretion as well as estradiol secretion from the ovary. Repeated oral administration of nigellar could improve these inhibited hormones, leading to their normalization to an extent. However, these improving effects of nigellar may be due to a neural opioid receptor affinity, which could adjust the catecholaminergic pathways of fertility hormones at the level of hypothalamus. These results assumed that thyroid disorders disrupted ovarian cycle and related hormones that caused infertility and co-therapeutic potential of nigellar together with the chemotherapy of thyroid disorders have some medical importance in improving fertility, and so it can be recommended.