What is an ACTH level?
An ACTH test or ACTH level measures how much ACTH (adrenocorticotropic hormone) is in your pet's blood/serum in the blood. ACTH is a hormone produced by the pituitary gland in the brain. ACTH regulates the production of cortisol (a stress hormone) by the adrenal glands. Endogenous ACTH is produced internally, whereas synthetic ACTH is used for testing purposes.
If blood cortisol levels are low, the pituitary gland secretes more ACTH, which causes more cortisol to be produced. If there is a high amount of cortisol in the blood, the pituitary gland reduces the amount of ACTH it secretes, such that less cortisol is produced (a negative feedback system).
What can the measurement of ACTH tell us?
The measurement of endogenous ACTH may be used to diagnose which type of Cushing's disease (hyperadrenocorticism) your pet has.
- If Cushing's disease is caused by excessive pituitary production of ACTH, the amount of ACTH in the blood is normal or high.
- If Cushing's disease is caused by an adrenal tumor or cortisol-containing medications, the amount of ACTH in the blood is low (due to the negative feedback system described above).
Can ACTH alone diagnose Cushing's disease?
No, ACTH alone cannot diagnose Cushing's disease. In both healthy and diseased pets, ACTH is released from the pituitary gland in an episodic or pulsatile (up and down) fashion. Therefore, the concentrations of ACTH found in healthy pets and pets with the different types of Cushing's disease may overlap and may not allow a clear differentiation of the cause of the disease. In addition, stress may cause periodic increases in ACTH concentrations.
For this reason, ACTH values are most useful in determining the type of Cushing's disease present once the disease has been confirmed through other laboratory tests.
