Serum hormones in response to estradiol and (OR) progesterone in ovariectomized cows after thyroidectomy

Abstract

To evaluate the effect of gonadal steroid treatment and thyroidectomy on concentrations of gonadotropins and thyroid-stimulating hormone in the bovine, nonlactating Holstein cows were either thyroidectomized and ovariectomized (THYOVEX; n=6) or ovariectomized only (OVEX; n=4), and subsequently treated with no gonadal steroids (control), estradiol-17β (E 2), progesterone (P 4), or P 4+E 2 in a 2 × 4 factorial experiment. Averaged across steroid treatments, baseline concentrations of luteinizing hormone (LH; P < .05) and follicle-stimulating hormone (FSH; P <.10) were higher in THYOVEX cows than in OVEX cows. Pulse frequencies and amplitudes of LH and FSH did not differ between THYOVEX and OVEX cows. Secretion of TSH was pulsatile and all concentrations and pulsatile characteristics of TSH were increased (P < .05) in THYOVEX compared to OVEX cows. Treatment with E 2 and P 4 decreased (P < .05) baseline concentrations and magnitude of LH pulses, whereas P 4+E 2 increased (P < .01) pulse frequency of LH and FSH. Amplitude of LH and FSH pulses were not affected by treatment with either steroid. Treatment with P 4+E 2 decreased (P < .05) baseline concentrations of TSH, whereas pulse frequency, and magnitude and amplitude of TSH pulses were not altered by treatment with steroids. Mean concentrations of LH and FSH were similar during 48 hr after termination of E 2 and P 4+E 2 treatments, but concentrations of TSH were higher (P = .06) after P 4+E 2 than after E 2. Secretion of TSH showed a diurnal variation, with the lowest concentrations in the morning and highest in the afternoon. These results indicate that thyroidectomy influenced secretion of gonadotropins in OVEX cows.