Precocious onset of spermatogenesis in juvenile carp (Cyprinus carpio L., Teleostei) following treatment with low doses of L-thyroxine.

: Two strains of carp (Cyprinus carpio L.), a heterogenic and an all male isogeneic, were used as experimental models to assess the effects of low doses of L-thyroxine (T4) on testicular development in juvenile fish. In four experiments (two with each strain) precocious onset of spermatogenesis was induced following exposure to 15 micrograms T4 l-1, but not to 8 micrograms or 30 micrograms T4 l-1. No significant increase in body weight and testis diameter (week 16 excepted) or numbers of primary spermatogonia was observed. Total T4, measured in the isogeneic strain, increased significantly in the serum of experimental specimens within two weeks after the onset of exposure, evidencing a readily absorption of the exogeneous T4. The contents of free T4 and total triiodothyronine (T3) in the serum were measured too, showing different levels in the two experiments, as also was observed with total T4. The content of free T3, which is formed in the cells of peripheral tissues by deiodination of T4 and which supposedly exerted the observed effect on spermatogenesis, could not be determined by the used standard Amerlite assays. It is hypothesized that T3 enhances in males the activity of gonadotropic hormones, which in turn will stimulate steroid production, similar to findings in females.