Low ACTH is seen with Addison's disease (rare autoimmune disease) and adrenal insufficiency (also called adrenal fatigue).
ACTH stimulates the adrenal glands to make cortisol (also known as the stress hormone). If you have a low level of ACTH then your adrenals make too little cortisol.
Cortisol is a hormone that is vital for health and has many functions which include helping to regulate blood pressure and the immune system, balance the effect of insulin in regulating the blood sugar levels and helping the body respond to stress.
From the website - Adrenal Recovery Fatigue:
"Adrenal Function Tests You Can Do at Home
ADRENAL FUNCTION TEST #1-Postural Hypotension:
Postural hypotension (also known as orthostatic hypotension) is a drop in blood pressure that occurs upon rising from a horizontal position. It is commonly expressed as a feeling of dizziness or lightheadedness, a "head rush", or "standing up too fast".
To do this test, you will need a blood pressure cuff. Lie down and rest for 5 minutes. Take a blood pressure reading while still horizontal. Then, stand up and take another reading.
Normally, your blood pressure should rise 10-20 points. If it drops, particularly by 10 points or more, hypoadrenia is indicated. Generally, the bigger the drop, the greater the adrenal insufficiency.
It should also be mentioned that low blood pressure in general is also an indicator of exhausted adrenals when present in conjunction with the other symptoms of adrenal gland fatigue.
ADRENAL FUNCTION TEST #2-Iris Contraction Test
For this test you will need a weak flashlight or penlight, and a mirror. In a dark bathroom or closet, wait a minute for your eyes to adjust to the dark. This will allow your pupils to dilate (open) fully. Then, shine the flashlight into your eyes, and watch the reaction of your pupils for at least 30 seconds.
The light should cause your iris to contract, making your pupils (the dark spot in the center of your eye) smaller. Normally, they should stay that way, but if you have adrenal gland fatigue, the iris will be weak and will not be able to hold the contraction, it will either waver between contracted and relaxed, or will contract initially, but then open up after 10-30 seconds.
As with the postural hypotension test, the degree to which you "fail" this test is an indicator of the degree of adrenal insufficiency you are experiencing.
ADRENAL FUNCTION TEST #3-Sergent's Adrenal White Line
With your fingernail or the dull end of a spoon, draw a line across your belly. In moderate to severe cases of adrenal fatigue, the line will stay white, and even get wider over the course of time, while a "normal" reaction would be for the line to almost immediately turn red.
This test has historically been used to indicate severe adrenal fatigue and Addison's Disease, milder cases of adrenal fatigue may not exhibit this sign."
I read breathily about Addison while googling but my cortisol, while not high, is still normal and my ACTH was up the next test. However they’re in the low. Last tests showed ACTH 8.7 (7.2-63.3 range) and cortisol 7.5 (5.0-17.9).