The effects of therapy with rifampicin and isoniazid on basic investigations for Cushing's syndrome.

Rifampicin(R) is a potent enzyme inducer which is now widely used to treat many conditions. We have investigated its effect on adrenal function tests in 18 adults with tuberculosis on rifampicin (R) and isoniazid (INH) as in-patients. Midnight cortisol was above 250 nmol/l in 11 patients with a mean +/- (SD) of 340 +/- 193 nmol/l. The morning serum cortisol following 1 and 4 mg dexamethasone overnight was abnormal in 88.9 per cent and 83.3 per cent respectively. The respective mean values +/- (SD) were 350 +/- 179 and 336 +/- 279 nmol/l. The mean 24-hour urinary free cortisol +/- SD was 513 +/- 375 nmol and was above normal in 6 (33.3 per cent) patients. These results show that midnight cortisol, dexamethasone suppression tests and 24-hour urinary free cortisol are misleading in patients with tuberculosis on (R) and should not be employed for investigating such patients for Cushing's syndrome.