Thyroxine and triiodothyronine can be measured as free thyroxine and free triiodothyronine , which are indicators of thyroxine and triiodothyronine activities in the body. They can also be measured as total thyroxine and total triiodothyronine , which also depend on the thyroxine and triiodothyronine that is bound to thyroxine-binding globulin . A related parameter is the free thyroxine index , which is total thyroxine multiplied by thyroid hormone uptake , which, in turn, is a measure of the unbound thyroxine-binding globulins.  Additionally, thyroid disorders can be detected prenatally using advanced imaging techniques and testing fetal hormone levels. 
In addition, maintaining normal or above-normal levels of thyroid hormone in the blood can help to reduce the amount of thyroid stimulating hormone (TSH) made by the pituitary gland. TSH is a regulatory hormone that stimulates the growth of the thyroid gland, and may also stimulate thyroid cancer cells. Normally, the pituitary gland makes more TSH when levels of thyroid hormone are low, so increasing these levels can send a signal to reduce production of thyroid stimulating hormone, and potentially help to keep some thyroid cancers from recurring.
In response to Marlene’s post: If you really have thyroid disease (Hashimoto’s) there is no vitamin supplement or diet that is going to effectively treat or reverse this condition, although there are supplement that may help to support thyroid function. Hashimoto’s is a progressive autoimmune disorder and it can cause wild fluctuations, causing you to swing from hyper to hypo until it finally just goes into a steady decline in hormone output. There are negative health risks when you let this go on too long without proper treatment. You must have prescription medication, whether it’s a “natural” thyroid drug such as Armour, compounded dessicated thyroid, or a synthetic hormone such as Synthroid (levothyroxine) and/or Cytomel (liothyronine.)
I am very athletic and helath concious and have been for most of my life. I was diagnosed with Hashimoto’s a few years ago and now that I’m being properly medicated (I take a combo of Synthroid and Cytomel) I look and feel better than I have in years! My athletic performance has increased, my hair is healthier, my skin is healthier, I never get sick anymore, I think more clearly, I’m happier, I have normal periods and I am no longer hanging onto an extra 5-10 lbs that wouldn’t go away no matter how hard I worked or how clean my diet was.