Effect of transient early hyperthyroidism on onset of puberty in Suffolk ram lambs.

The thyroid function and sexual development of eight 6-week-old Suffolk ram lambs were studied. The lambs were divided into either control or treatment groups and housed indoors. From 6 to 12 weeks of age, four lambs in the treatment group received 15 mg kg(-1) body weight per day of 6-propyl 2-thiouracil orally to suppress normal thyroid function. During the same period, thyroxine and tri-iodothyronine were injected s.c. at the rate of 8 and 16 microg kg(-1) body weight per day, respectively, to induce a hyperthyroid state. Four control lambs received sham injection and oral excipient. Concentrations of thyroxine, tri-iodothyronine, FSH, testosterone and insulin-like growth factor I were determined in blood collected by indwelling jugular catheters once a week, every 20 min from 09:00 to 15:20 h. Scrotal circumference was recorded each week. Semen collection was attempted by electro-ejaculation between weeks 17 and 36. Lambs were castrated at week 36 and testicular histology was examined. During the treatment period only, the concentration of thyroid hormones was higher in treated lambs than in controls (P < 0.05). From week 6 to week 9 only, concentrations of FSH in treated lambs were lower than in controls (P < 0.05). Insulin-like growth factor I concentrations were lower in treated lambs than in controls from week 10 to week 13 (P < 0.05). Frequency of testosterone pulses was higher (P < 0.01) in the treated lambs but concentrations were similar in the control and treated lambs throughout the experiment. Scrotal circumference was greater in treated lambs from week 26 to week 36 (P < 0.05). Treated lambs produced viable spermatozoa earlier than did control lambs. At week 36, sperm concentration in treated lambs was higher than in controls (P < 0.01) but semen volumes were similar (P > 0.1). Diameter of the seminiferous tubules in treated lambs was larger than in controls (P < 0.05). In conclusion, transient neonatal hyperthyroidism decreased FSH and insulin-like growth factor I concentrations temporarily, increased testosterone pulses and sperm production and advanced puberty in Suffolk ram lambs.