Adrenal insufficiency is diagnosed by confirming an inappropriately low cortisol secretion, and it is then classified as primary, secondary, or tertiary.
Confirming cortisol deficiency
The diagnosis of adrenal insufficiency rests on the laboratory confirmation of inappropriately low cortisol secretion. Three tests can provide it:
| Test | Interpretation |
|---|---|
| Basal morning serum cortisol concentration | Physiologically the highest serum cortisol is in the morning, between 6 and 8 am, with a peak between 10 and 20 mcg/dl; a morning level less than 10 mcg/dl makes adrenal insufficiency suspect, and less than 3 mcg/dl is highly suggestive. |
| Morning salivary cortisol concentration | Adrenal insufficiency is ruled out if the morning salivary cortisol is more than 5.8 ng/ml, and less than 1.8 ng/ml is highly suggestive. |
| Urinary free cortisol (UFC) | Not generally recommended, because it can be low-normal even in the early stages of adrenal insufficiency. |
In some cases the diagnosis is very straightforward: the cortisol is very low and the ACTH is very high at the same time, which clearly indicates primary adrenal insufficiency. Often it is not. A patient may sit in the gray zone, with a cortisol at the lower limit of normality and an ACTH at the upper limit; that pattern could be either the primary or the secondary form, so a stimulation test based on the injection of ACTH is needed.
Stimulation tests
For screening and confirmation, the standard-dose ACTH stimulation test measures the ability of the adrenal gland to produce cortisol in response to an intravenous injection of 250 mcg of ACTH, with cortisol measured at 0, 30 and 60 minutes. The test is negative if the peak cortisol is higher than 18-20 mcg/dl, which excludes primary and almost all cases of secondary adrenal insufficiency. If the result still leaves the diagnosis in the gray zone, a test that stimulates CRH and ACTH release more strongly is needed, such as insulin-induced hypoglycemia, since hypoglycemia is the most powerful stimulus for the release of CRH.
The standard dose has one weakness: it can under-diagnose partial adrenal insufficiency. Because 250 mcg of ACTH is a very high stimulus, it can drive even a partially damaged gland to secrete a normal-looking amount. The low-dose ACTH stimulation test instead gives a smaller stimulus, injecting 1 mcg of ACTH intravenously at the time of the lowest physiological cortisol concentration, around 14:00, and measuring serum cortisol at 0, 10, 15, 20, 25, 30, 35, 40 and 45 minutes afterwards. If cortisol at any time after the low dose reaches higher than 18 mcg/dl, adrenal insufficiency is ruled out. In partial adrenal insufficiency, and in secondary and tertiary adrenal insufficiency especially in the early stages, the low-dose test has higher sensitivity.
Separating the three forms
Once cortisol deficiency is confirmed, the pattern of the axis hormones places the lesion:
| Parameter | Primary adrenal insufficiency | Secondary and tertiary adrenal insufficiency |
|---|---|---|
| Morning serum cortisol | low | low |
| ACTH | elevated (normal at 8:00 am is about 20-52 pg/ml) | reduced |
| Aldosterone | low | normal |
| Serum renin activity | elevated | normal |
| Potassium | high | normal |
| Sodium | low | low or normal |
For the differential diagnosis between secondary and tertiary adrenal insufficiency, the CRH test is performed: in secondary adrenal insufficiency there is no increase of ACTH after the CRH test, whereas in tertiary adrenal insufficiency there is an exaggerated increase of ACTH upon injection of CRH, without an increase of cortisol.
Other, less used tests can confirm the diagnosis in some cases: the prolonged ACTH stimulation test, the insulin-induced hypoglycemia test (usable because the most powerful stimulus for the release of CRH is hypoglycemia), and the CRH test.
Workup of the underlying cause
Once the level of the lesion is known, the search turns to its cause. In confirmed primary adrenal insufficiency, autoantibody screening shows positivity for autoantibodies against 21-hydroxylase in more than 90% of Addison’s disease cases, which is the autoimmune form. The VLCFA test, which measures the very long chain fatty acids that accumulate in adrenoleukodystrophy, confirms that disease. In patients with primary adrenal insufficiency and a negative autoantibody test, CT imaging of the adrenal glands is performed. In confirmed secondary or tertiary adrenal insufficiency, MRI of the hypothalamus and pituitary gland is performed. The level of the lesion and its cause together determine which hormones have to be replaced.