SERUM HORMONES IN RESPONSE TO ESTRADIOL AND (OR) PROGESTERONE IN OVARIECTOMIZED COWS AFTER THYROIDECTOMY

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 an d ovariectomized (THYOVEX; n=6) or ovariectomized only (OVEX; n=4), an d subsequently treated with no gonadal steroids (control), estradiol-1 7 beta (E(2)), progesterone (P-4), or P-4+E(2) in a 2 x 4 factorial ex periment. 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. Purse fr equencies and amplitudes of LH and FSH did not differ between THYOVEX and OVEX cows. Secretion of TSH was pulsatile and all concentrations a nd pulsatile characteristics of TSH were increased (P < .05) in THYOVE X compared to OVEX cows. Treatment with E(2) and P-4 decreased (P < .0 5) baseline concentrations and magnitude of LH pulses, whereas P-4+E(2 ) increased (P < .01) pulse frequency of LH and FSH. Amplitude of LH a nd FSH pulses were not affected by treatment with either steroid. Trea tment with P-4+E(2) decreased (P < .05) baseline concentrations of TSH , whereas pulse frequency, and magnitude and amplitude of TSH pulses w ere 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 after noon. These results indicate that thyroidectomy influenced secretion o f gonadotropins in OVEX cows.