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How Accurate (and useful) is an FT3 Bloodtest?

People often ask me questions like:

"Why do I feel terrible when my FT3 is actually mid range or even a bit above mid range?"
"My doctor says that my FT3 is fine, and I don't need T3 or Natural Desiccated Thyroid but I have all the symptoms of hypothyroidism. How can that be?"
"If my FT3 is ok and my rT3 isn't that high but I feel terribly hypothyroid and have done since I had Hashimoto's thyroiditis then could T3 really help me?"

This is an interesting article that was written in 2006 by a retired chief bio-medical scientist called Nick Welsh.

He believes that some people may have Addison's disease or partial adrenal insufficiency and yet still have good levels of cortisol when tested in blood (either through a standard 8:00am morning cortisol test OR a standard 8:00am Synacthen test/ACTH Stimulation test). He goes on to explain that some groups of people (including those with Hashimoto's thyroiditis) are prone to elevated cortisol binding globulin (CBG). CBG bings much of the free cortisol. 

So much has been written on the subject of 'resting the adrenal glands' when a thyroid patient is suffering from partial adrenal insufficiency (sometimes called 'adrenal fatigue').

I tend not to use the term 'adrenal fatigue' any more as it is not medically accepted or even understood by many doctors and endocrinologists. I try to use hypocortisolism now, as it does not imply the cause of the cortisol issue.

Here is an interesting short report by thyroid patient Michelle Weiler about her own experience with bio-identical progesterone use and its effect on blood testing of hormones like cortisol.

Blood Testing on Progesterone Supplementation: What it Can and Can't Tell Us - Michelle Weiler