Influence of hypo- or hyperthyroidism on ovarian function in Brahman cows

Multiparous Brahman cows (n = 21) were randomly assigned during late fall within BW and body condition score (BCS) to receive either 3.0 mL of corn oil (C; n = 7), 3.0 mg/(cow.d) triiodothyronine (T3) s.c. in 3.0 mL of corn oil (HYPER; n = 7), or 4.0 mg/(kg.d) 6-n-propyl-2-thiouracil (PTU; fed with concentrate) plus 3.0 mL/d corn oil (HYPO; n = 7). Water, minerals, and Coastal bermudagrass hay were available free choice, and all cows received 3.2 kg.cow-1.d-1 of 5:1 corn:soybean meal concentrate. The feeding period extended through three normal estrous cycles. Blood samples were collected weekly during the first and second estrous cycle, or until d 42 for anestrous cows, and daily throughout the third cycle. Also, between d 9 and 14 of the third cycle, or after d 35 in anestrous cows, intensive samples were collected at 2-h intervals for 24 h. Serum T3, thyroxine (T4), and progesterone (P4) were measured in weekly and intensive samples, and cortisol, insulin, GH, and thyroid-stimulating hormone (TSH) were measured in intensive samples. The altered thyroid status of HYPER and HYPO cows was evident (P .001) during the third estrous cycle in mean daily T3, T4, and intensive TSH (P .001) concentrations. Changes in BW and BCS were influenced by treatment (P .001). A greater (P .001) proportion of HYPER cows exhibited abnormal cycle length, and three of seven cows became anestrous. For cows that continued normal cycles, treatment did not affect (P .05) the number of follicular waves, diameter of the dominant follicle, diameter of the ovulatory follicle, or P4 profiles during the third cycle. Insulin and GH concentrations did not differ (P .05) among treatments in intensive samples, but, mean cortisol was greatest (P .02) in HYPER cows. For Brahman cows that maintained normal estrous cycles, induced hyperor hypothyroid status did not influence ovarian function