Interrelation between thyroid hormones and onset of puberty in ram lambs

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Fallah-Rad, Amir Hooshang
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This project had two phases. The aim of th first phase of the study was monitoring concentrations and profiles of testosterone, thyroxine (T$\sb4$), tri-iodothyronine (T$\sb3$), insulin-like growth factor-I (IGF-I), glucose and testosterone pulse frequency during the peripubertal period and to investigate the relationship between these hormones and testicular function in the Suffolk (Sfk) and the more prolific Outaouais (Out) ram lambs. Average daily gain (ADG), relative growth (RG), scrotal circumference (SC), body weight (BW), gonadosomatic index (GSI) and blood glucose of the two breeds were similar throughout the experiment. Sperm cell concentration was higher in Out than in contemporary Sfk ram lambs. Mean testosterone concentration and pulse frequency during the 6 h period in each sampling day, T$\sb4$, T$\sb3$ and IGF-I were higher in Out than Sfk throughout the experiment. Analysis of the 20 min samples revealed no episodic release of IGF-I in both breeds. In conclusion, Out ram lambs had attained pubertal testicular function at an earlier age than Sfk ram lambs. Higher T$\sb4$, T$\sb3$, testosterone concentrations, testosterone pulse frequencies, and IGF-I in Out than Sfk may be associated with higher prolificacy and sperm concentration in the Out ram lambs. The second phase was designed to investigate the influence of transient neonatal hyper- and hypothyroidism on the profiles of testosterone, follicle stimulating hormone (FSH), IGF-I, testicular development, function and onset of puberty in Sfk ram lambs. Ram lambs were divided into four groups: control, PTU40, PTU55 and PTU+H. In the PTU+H group the lambs were induced hyperthyroid for 40 d. In the PTU40 and PTU55 the lambs were transiently hypothyroid for 40 and 55 d respectively. In all the treatment groups the level of FSH was lower during the treatment period. In the PTU55 group, FSH levels remained low after the treatment which is probably the result of direct effects of changes in the thyroid state on the higher brain centers. In both hypo- and hyperthyroid lambs, level of testosterone and pulse frequency did not change. Body weight and feed consumption in the treated animals were not different from those of controls. Hypothyroidism did not change testicular size, but decreased sperm production. Hyperthyroidism, on the other hand, increased sperm concentration and testicular size. Concentration of IGF-I was lower in both hypo- and hyperthyroid lambs, but only during the treatment period, indicating the negative impact of thyroid manipulation on IGF-I. In conclusion, the above results show the critical role of thyroid hormones on the function and development of the male reproductive system early in life and that hypo- and hyperthyroidism can change the profile of the reproductive processes in male sheep.