What Is the Dexamethasone Suppression Test?

Dexamethasone is one of the most biologically active hormones in synthetic glucocorticoids. It only needs a small amount to achieve a similar effect to natural cortisol. Because of its small amount, its concentration in blood is very low. It is difficult to measure by conventional radioimmunoassay, so it has no effect on the determination of autocortisol secretion. This test uses the characteristics of dexamethasone to understand the hypothalamus through its inhibitory effect on the adrenocorticotropic hormone and corticotropin-releasing hormone secreted by the pituitary and hypothalamus, and the degree to which the secretion of adrenocortical hormone is reduced. -Whether the pituitary-adrenal axis function is higher than normal, and its possible lesion is in that organ.

Dexamethasone inhibition test

The dexamethasone test understands the hypothalamus-pituitary-adrenal gland through the inhibitory effect of dexamethasone on the pituitary and hypothalamic secretion of adrenocorticotropic hormone and corticotropin-releasing hormone, and the degree of reduction of adrenocortical hormone secretion. Whether the shaft function is higher than normal and its possible lesion is in that organ.

It is divided into 4 ways according to different doses and methods of dexamethasone administration, and the clinical significance is also different.

Midnight one piece dexamethasone test

The method was to determine blood cortisol by taking blood at 8 am on the day of the control, and taking 0.75 mg of dexamethasone orally (up to 1-1.5 mg in obese persons) at 24 o'clock that night, and then taking blood to measure cortisol at 8 am the next day. Analysis of results: The control day cortisol level should be within the normal range (8 o'clock, 184.9-59.4 nanomoles / liter), and should be reduced to less than 82.8 nanomoles / liter after taking the drug.

The clinical significance of this test is a screening test to understand whether the adrenal cortex function is normal. If the dose is> 82.8 nanomoles / liter, a small dose of dexamethasone inhibition test should be further selected.

Low-dose dexamethasone inhibition test

The test method was oral dexamethasone 0.5 mg, once every 6 hours for 2 days. Urine free cortisol was measured in urine for 24 hours before and on the second and fifth days of medication. Blood was drawn before and on the third day at 8 am to determine corticotropin and cortisol.

Analysis of the results: After taking the drug, the free cortisol in the urine should be suppressed to <69 nanomoles / 24 hours, and the blood cortisol should be <82.8 nanomoles / liter.

Clinical significance: Such as cortisol and urinary free cortisol are not inhibited, suggesting the existence of hypercortisolism.

High-dose dexamethasone inhibition test

The test method was oral dexamethasone 2 mg, once every 6 hours for 2 days. The method of urine retention and blood test is the same as the low-dose dexamethasone inhibition test.

Analysis of results: After taking the drug, the blood and urine cortisol values fell below 50% of the control value as a response.

Clinical significance: used to identify the etiology of Cushing syndrome. In patients with Cushing syndrome, hypercortisolism caused by adrenocortical tumors has largely inhibited the secretion of pituitary corticotropin, and the administration of exogenous glucocorticoids will not affect the adrenal cortex. How much the hormone secretion affects, blood, urine cortisol also change little. The high-dose dexamethasone has a certain inhibitory effect on Cushing's disease caused by pituitary lesions, which reduces the secretion of pituitary adrenocorticotropic hormone and the corresponding decrease in cortisol secretion, and the inhibition rate can reach> 50%.

Medium dose dexamethasone inhibition test

The method was oral dexamethasone 0.75 mg, once per 6 hours for 5 days. The urine was checked for 17-ketosteroids 24 hours a day before and on the third and fifth days.

Analysis of the results: After taking the drug, compared with the control value, the urine 17-ketosteroid was inhibited by more than 50% as normal.

Clinical significance: It is mainly used to understand whether the sex hormones secreted by the adrenal glands are normal. If the basal urine 17-ketosteroid is higher than normal, it can drop by more than 50% after being inhibited by dexamethasone, suggesting that the elevated 17-ketosteroid originates from the adrenal gland; if less than 50%, it may be from the gonads or other parts.

Note: Normal value

Small-dose split medication method: Normal people take dexamethasone for two consecutive days (0.5mg, once every 6h for a total of 8 times; or 0.75mg, once every 8h for a total of 6 times), on the second day of medication The measured urine 17-hydroxycorticosteroids were significantly lower than the control day before taking the medicine, with an average decrease of 74.4%, ranging from 39.8% to 100%. About 70% of normal people have the lowest inhibition below 3mg / 24h.

Low-dose once-midnight administration (using urine 17-hydroxyl as an indicator): Normal people take 0.75mg of dexamethasone every night at 12 o'clock. 2 consecutive days. The 17-hydroxy content in the 24-h urine collected after the second administration and after the medication was reduced by more than 50% compared with the control value before the medication.

Low-dose once-mid-dose regimen (using cortisol as an indicator): After taking 0.75 or 1 mg of dexamethasone at 12 noon, the plasma corticosteroids in the next morning dropped to less than 50% of the control value before medication.

High-dose suppression test: Normal people take large doses of dexamethasone continuously for 2 days (2mg, once every 6 hours for a total of 8 times). The urine 17-hydroxysteroid content measured on the second day after taking the drug (that is, after taking 4 times) should be less than 50% of the control value.