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Gonadal Testing Guidelines

Determination of gonadal status in male horses - detection of cryptorchid males

Testosterone

Testosterone concentrations in the stallion vary with season, with a mean of 3 nmol/L in the non-breeding season, rising to 12 nmol/L at the height of the breeding season. Geldings have low or undetectable concentrations (less than 0.19 nmol/L) of testosterone. A cryptorchid is confirmed if the serum testosterone concentration is greater than 0.3 nmol/L.

Baseline testosterone concentrations are not significantly different between stallions and unilateral cryptorchid horses. Bilaterally cryptorchid horses have a lower baseline concentrations and most commercially available assays are not sensitive enough to detect these low levels. A single serum testosterone test is therefore not routinely recommended for diagnosing cryptorchidism as false negatives or equivocal results are common.

Oestrone sulphate

The equine testes produces ten times the quantity of oestrogen compared with testosterone and it is therefore much easier to identify a cryptorchid from the gelding by measuring serum oestrone sulphate. This test is not recommended in horses less than three years of age and donkeys. No data is available on the value of oestrone sulphate in miniature ponies. Reference ranges are as follows:

Intact stallions 10 - 100 ng/ml

Probable castrate < 1 ng/ml (false rig)

Equivocal 1 - 5 ng/ml

Cryptorchid >5 ng/ml (true rig)

It is recommended that those animals with oestrone sulphate concentrations in the equivocal range be tested by hCG stimulation.

Cryptorchids are unilateral, bilateral, inguinal and abdominal and their oestrone sulphate concentrations may vary and overlap with stallions and castrates. Diagnosis therefore also relies on other factors including behaviour, palpation, ultrasonography, etc.

Confirmation in young horses (<three years of age) and in donkeys requires a hCG stimulation test:

- take serum sample and inject 6000 IU hCG IV

- take serum sample 30 to 120 minutes later

- measure serum testosterone in both samples

A true rig will show a testosterone increase from 0.3 - 4.3 nmol/L to a post-stimulation concentration of 1 - 12.9 nmol/L.

A castrate will have testosterone concentration <0.19 nmol/L.

Anti-Mullerian hormone (AMH)

Serum AMH may be used to differentiate cryptorchid and castrated male horses.

Alpacas and Llamas

A single testosterone measurement is often all that is required. If equivocal results are found, then a hCG stimulation test can be used. For this test, serum testosterone concentrations are measured in two blood samples collected before and 18 hours after IM administration of 5000 IU hCG. Note that alpacas reach sexual maturity at 10 to 12 months of age and llamas at 12 to 24 months.