Short Tetracosactrin (Synacthen) Test
- The test should be done when the patient has recovered from acute illness, as hydrocortisone (but not fludrocortisone) must be stopped for 24h before the test. The patient should be resting quietly but need not fast prior to the test. Tetracosactrin may exacerbate bronchospasm in those with asthma. Oestrogens, for example in oral contraceptives, should be stopped six weeks before the test.
- Give 250μgm of tetracosactrin IV or IM before 10am. Measure plasma cortisol immediately before, and 30 and 60 min after the injection. Baseline plasma ACTH should also be measured.
- With normal adrenal function, the baseline plasma cortisol is over 140nmol/L, and the 30 or 60-min level is over 500nmol/L and at least 200nmol/L above the baseline level. (note: cut off values are dependent on the assay used - check local values with your laboratory)
- In patients with primary hypoadrenalism, tetracosactrin does not stimulate cortisol secretion, because the adrenal cortex is already maximally stimulated by endogenous corticotropin. In severe secondary hypoadrenalism, plasma cortisol does not increase because of adrenocortical atrophy. However, in secondary hypoadrenalism which is mild or of recent onset, the test may be normal.
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