Androgen and oestrogen response to a single injection of hCG in cryptorchid horses.

Androgen (testosterone and androstenedione) and oestrogen (oestradiol -17 beta and oestrone) concentrations were measured by radio-immunoassay in the peripheral plasma of two geldings (five-years-old), three bilateral cryptorchids (two, two and a half, and five-years-old) and three normal intact stallions (four, five and five and a half-years-old) before and after a single injection of 10,000 iu human chorionic gonadotrophin (hCG). In the stallions, hCG administration resulted in an immediate sharp increase of conjugated oestrogens and a more gradual increase of unconjugated androgens. In the cryptorchids, the unconjugated androgens increased following a similar pattern to that observed in the stallions, but reached lower peak values, whereas the conjugated oestrogens showed only a very slight increase. In the stallions and cryptorchids, the maximum oestrogen levels were reached two days after injection, whereas the maximal levels for androgens were reached a day later. In the geldings, hCG injection had no effect on plasma steroid levels. It is suggested that the measurement of unconjugated androgens (testosterone or/and androstenedione) before and three days after intravenous injection of 10,000 iu hCG may prove useful for the diagnosis of cryptorchidism or exploration of testicular function in stallions.