Short Synacthen test for CAH
This is performed for the investigation of congenital adrenal hyperplasia (CAH) in children and adults.
The Synacthen test gives unreliable results within 2 weeks of pituitary surgery.
Adrenal glucocorticoid secretion is controlled by adrenocorticotrophic hormone (ACTH) released by the anterior pituitary. This test evaluates the ability of the adrenal cortex to produce cortisol after stimulation by synthetic ACTH (tetracosactide: Synacthen ®). In subjects with enzyme deficiency in the steroid synthetic pathway, cortisol may, or may not, be adequately secreted. However, there is excessive secretion of the precursor steroids before the defective enzyme. The commonest form of CAH is due to deficiency of 21-hydroxylase and in these subjects increased secretion of 17 OH-progesterone can be detected.
There are rare reports of hypersensitivity reactions to Synacthen particularly in children with history of allergic disorders.
There are no dietary restrictions for this test. This test should be performed in the morning as diurnal variations in 17 OH-progesterone have not been established in CAH.
Hydrocortisone should be omitted on the morning of the test.
Prednisolone shouled be stopped 24 hours before the test and recommenced after (unless instructed to the contrary).
- 2 plain tubes and 1 EDTA tube
- 250 microgram Synacthen (1 vial)
- the dose for children is 36 microgram/kg body weight up to a maximum of 250 microgram
take 3 mL blood for cortisol , 17 OH-progesterone and ACTH
inject Synacthen iv
take further sample for cortisol and 17 OH-progesterone
A normal cortisol response would be indicated by a rise in the cortisol concentration of the 30 min sample to greater than 600 nmol/L.
There are marked variations in 17 OH-progesterone throughout the menstrual cycle and "normal" values cannot exclude non-classical CAH. Heterozygotes for 21 OH-deficiency should have post-ACTH values of 17 OH-progesterone > 35 nmol/L (guide value). There is an overlap with normals and this test is not diagnostic.
Homozygotes for non-classical CAH may have normal baseline values for 17 OH-progesterone but will have exaggerated responses to ACTH of > 60 nmol/L (guide value).
Adult individuals with the rarer forms of late-onset CAH eg 3 beta hydroxylase or 11 hydroxylase have normal 17 OH-progesterone responses to ACTH.
Sensitivity and Specificity
- Eldar-Geva T, Hurwitz A, Vecsei P, Palti Z, Milwidsky A, Rosler A. Secondary biosymthetic defects in women with late-onset congenital adrenal hyperplasia. NEJM 1990;323:855-863.
- New MI, Lorenzen F, Lerner AJ, Kohn B, Oberfield SE, Pollack MS, Dupont B, Stoner E, Levy DJ, Pang S, Levine LS. Genotyping steroid 21-hydroxylase deficiency: hormonal reference data. J Clin Endocrinol Metab 1983;57:320-6
JHB 22 Sept 2011