Inhibin is a polypeptide hormone secreted by ovarian granulosa cells in females and testicular sertoli cells in males. It selectively suppresses the secretion of pituitary FSH and also has local paracrine actions in the gonads.

The most common ovarian tumour in the mare is the granulosa cell tumour (GCT). Clinical diagnosis of GCTs is usually based on behavioural abnormalities such as prolonged anoestrus, stallion like behaviour, persistent oestrus and nymphomania and are often difficult to handle. Unilateral ovarian enlargement with atrophy of the other ovary is also a common finding.

Testosterone is elevated in approximately 54% of mares with GCTs and usually only those showing stallion-like behaviour. Mares showing anoestrous or persistent oestrous may have normal testosterone concentrations. Progesterone concentrations are almost invariably low
(< 1.0 nmol/L) in mares with GCTs and as mares are usually acyclic, the detection of an elevated progesterone (>3.0 nmol/L) suggests that a GCT is unlikely to be present. Oestradiol levels are very variable and are not suitable as an aid to the detection of GCTs.

Inhibin concentrations have been found to be elevated above normal values in approximately 87% of mares with GCTs and therefore appears to be a much more accurate indicator of the presence of a GCT than testosterone alone. The measurement of progesterone, testosterone and inhibin is suggested and we offer this as a GCT profile.