As summarized in Antech News (September 2001), serum 17-hydroxyprogesterone (OHP) concentrations can be helpful in the diagnosis of atypical cases of hyperadrenocorticism (HAC), namely, those with normal ACTH stimulation and LDDS test results. Some dogs with HAC could have deranged steroid production pathways, such that certain steroid precursors may be abnormally increased and responsible for the presenting clinical signs, whereas cortisol concentrations are normal.

A retrospective study evaluated 7 dogs with clinical signs and routine laboratory data suggestive of HAC. Four dogs had normal ACTH stimulation and LDDS tests; 3 were ultimately determined to have PDH and 1 had an adrenal tumor HAC. Three dogs had normal ACTH stimulation tests and low cortisol concentrations throughout their LDDS tests. All 3 dogs had adrenal tumors. While resting, OHP concentrations were inconsistently increased in these dogs, all 7 had exaggerated OHP concentrations in response to ACTH administration.

Elevated post-ACTH OHP (and possibly other adrenal sex hormones) may be useful in confirming a diagnosis in dogs with atypical HAC. Dogs suspected of having HAC, but with normal or low cortisol concentrations after receiving ACTH or low-dose dexamethasone, and no history of steroid administration, should be evaluated for a sex hormone-producing adrenal tumor with OHP testing and adrenal gland ultrasonography.

The test requires 50 µl of serum collected 1 hour post-synthetic ACTH or 2 hours post-ACTH gel administration. The test can be added on to a previously submitted post-ACTH serum sample, as it is not necessary to measure resting (pre-) OHP concentrations.