Effects of Thyroidectomy and Thyroxine Replacement on the Release of Luteinizing Hormone and Gonadotropin-Releasing Hormone in vitro

Abstract

The effects of thyroidectomy and thyroxine (T4) replacement on the release of luteinizing hormone (LH) and gonadotropin-releasing hormone (GnRH) in ovariectomized (Ovx) rats were studied. Immediately after ovariectomy, rats were thyroidectomized (Tx) or sham-Tx. The Ovx-Tx rats were injected subcutaneously with either saline or T4 (2 µg/100 g body weight) daily for 30 days before sacrifice. Sham-Tx rats were treated with saline only. Twenty hours after the last injection, the blood sample was obtained by decapitation. The excised anterior pituitary gland (AP) was bisected and incubated in vitro with or without 0.1 0.5, 2.5, and 50 ng GnRH at 37 °C for 4 h. The mediobasal hypothalamus (MBH) was bisected and incubated with or without the AP of Ovx donor rat in vitro. Concentrations of LH and GnRH in the medium and that of LH in the serum were measured by radioimmunoassay. LH in the serum of Tx rats was higher than that in the serum of sham-Tx and Tx-T4 rats. Thyroidectomy resulted in an increase of LH release by Ovx rat AP, stimulated with or without 0.1 and 50 ng GnRH, as well as in an increase of immunoreactive GnRH release from MBH of Ovx rats in vitro. After a 4-hour incubation with donor APs, the LH in the medium containing MBH obtained from Tx rats was significantly higher than that obtained from sham-Tx and Tx-T4 rats. LH concentrations, in both sera and media, as well as GnRH concentration in the media of euthyroid and T4-replaced Tx groups were nonsignificantly different. These results suggest that T4 is inhibitory to the basal and GnRH-stimulated LH release as well as to the release of GnRH in the absence of ovarian hormones.

Effects of Prochloraz or Propylthiouracil on the Cross-Talk between the HPG, HPA, and HPT Axes in Zebrafish

“…Effects of PTU on interactions between thyroid hormones and genes of the HPG axis in female zebrafish were primarily through the release of FSH and LH, which is supported by physiological study . The significantly greater concentrations of T, FSH, and LH, expression of steroidogenic genes (StAR, 3βHSD, and 17βHSD) along the HPG axis were observed after PTU exposure, while the expression of genes along the HPG axis in brain did not change.…”

Effect of hypothyroidism on the pituitary-gonadal axis in the adult female rat

“…In ovariectomized rats, thyroidectomy induced an increase in plasma LH (LaRochelle & Freeman 1974, Wang et al 1987, Ruiz et al 1988). In intact rats, however, thyroidectomy caused a decrease in basal LH (Bruni 1975, Ortega et al 1990 or had no effect on basal LH (Baksi 1973, Dunn et al 1976), while it decreased the pro-oestrous LH surge (Dunn et al 1976).…”