Insufficiency of thyroid hormones inhibits gonadotropin release and results in dysregulation of reproductive function, although the precise mechanism of this disrupted gonadotropin secretion remains unclear. Kisspeptin is a neuropeptide that strongly stimulates gonadotropin secretion and plays an important role in reproductive function. To examine the involvement of kisspeptin in the dysregulation of gonadotropin secretion in hypothyroidism, we investigated Kiss] mRNA expression and kisspeptin immunoreactivity in the hypothalamus of female rats treated with propylthiouracil (PTU). In the PTU-treated rats, serum thyroxine (T4) was significantly decreased, whereas thyroid stimulating hormone (TSH) levels were significantly increased. In addition, irregular estrus cycles were observed in PTU-treated rats. In situ hybridization and immunohistochemistry revealed significant reductions in the number of Kiss/mRNAexpressing neurons and kisspeptin-immunoreactive neurons in the arcuate nucleus (ARC) but not in the anteroventral periventricular nucleus (AVPV) of the PTU-treated rats. Although the serum levels of luteinizing hormone (LH) and estradiol (E2) were unaffected, serum prolactin levels were significantly increased after PTU treatment. These data indicate that kisspeptin expression in the ARC is suppressed under thyroid hormone insufficiency, suggesting that the dysregulation of reproductive function in hypothyroidism is caused by inhibition of kisspeptin neurons in the ARC. (C) 2016 Elsevier Ireland Ltd and Japan Neuroscience Society. All rights reserved.