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Ovine CRH is a potent stimulator of pituitary ACTH secretion in humans. The ovine CRH stimulation test is used to differentiate pituitary ACTH dependent Cushing's syndrome from ectopic ACTH secreting tumors and adrenal dependent Cushing's syndrome. It also helps to distinguish between hypothalamic and pituitary dependent adrenal insufficiency. The patient should be fasting and at bed rest for at least 40 minutes before the first blood samle is drawn. This test is usually performed by administering one ug/kg of body weight, or 100 ug, of ovine (sheep) corticotropin releasing hormone (CRH) intravenously at 20:00 hours. Plasma ACTH and serum cortisol levels are measured 15 minutes prior to starting the test, just before the CRH is injected, and 15, 30, 45, and 60 minutes after the injection. Peak ACTH levels (130% of baseline) occur at 30 to 60 minutes and peak plasma cortisol levels (100% of baseline) occur at 45 to 60 minutes. ACTH secretion is found in patients with hypothalamic CRH deficiency and lack of ACTH secretion is seen in patients with pituitary insufficiency. Patients with pituitary ACTH dependent Cushing's syndrome have normal to increased response of plasma ACTH and cortisol levels after ovine CRH administration, whereas patients with adrenal dependent Cushing's syndrome usually do not respond to ovine CRH. Specimen requirement is one SST tube of blood for cortisol and two lavender top tubes for ACTH. |
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