Influence of Hypo- or Hyperthyroidism on Ovarian Function in Brahman Cows1

Multiparous Brahman cows (n = 21) were randomly assigned during late fall within BW and body condition score (BCS) to receive either 3.0 mt of corn oil (C; n = 7), 3.0 mg/(cow.d) triiodothyronine (T-3) s.c. in 3.0 mt 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 T-3, thyroxine (T-4), and progesterone (P-4) 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 T-3, T-4, 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 P-4 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 hyper- or hypothyroid status did not influence ovarian function.