What follows is information from THE THYROID GLAND: A book for patients, that I got from the Am. Thyroid Foundation.
TSH is thyroid stimulating hormone, which is produced by the pituitary gland. In people without thyroid disease, this gland and its TSH acts as a thermostat to regulate the amount of thyroid hormone that goes into the body. If the thyroid is producing too much, the TSH sent into the body is lowered; if the thyroid isn’t producing enough hormone, more TSH is sent out. So bloodwork which measures TSH show, by inference, levels of thyroid hormone in the body.
“Currently it is possible to measure blood levels of TSH very accurately by supersensitive TSH assays (S-TSH). Because S-TSH levels relate so well to thyroid function, it is now thought that S-TSH is the best single screening test for thyroid function.”
There are two principle thyroid hormones, thyroxine is called T4 because each molecule of hormone has four iodine atoms. Triiodothyronine is called T3 because there are only three iodine atoms in each molecule. “Even though T3 has less iodine, it is about four times as strong as T4. Although there is much less T3 in the blood than T4, T3 is the active hormone within body cells. Most T3 in the cells comes from T4 by splitting off one atom of iodine.”
“FREE THYROID HORMONE: Thyroid hormone travesl in the blood from the thyroid gland to its destinations attached (or bound) to carrier proteins. Bound hormone is not active. More than 99% of the thyroid hormone in the blood is protein-bound. j The tiny unbound fraction is called “free” hormone. It is the free hormone that is active in the body tissues. When one tests thyroid function, measurements of bound rather than free ormone levels can lead to incorrect diagnoses.”
The author goes on to discuss the various thyroid function tests, and talks about how it is possible to be confused (no fooling) over the T3 designation for tests because there are two entirely different tests having this designation, and you have to know WHICH one you received: one measures the amount of T3 in the blood, and the other measures protein-binding of T3.
Hope this answers some of your questions (without causing too many more!)
Bobbi
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