Kresser: Thyroid Matters

What’s most important to understand about this condition is that, although it does involve low levels of T3 (the most active form of thyroid hormone), it is not caused by a problem with the thyroid gland. This is a crucial distinction and it’s what distinguishes Low T3 Syndrome from “garden-variety” hypothyroidism.
In this series we’re going to discuss 1) what causes Low T3 Syndrome, 2) it’s clinical significance, and 3) if it should be treated, and if so, how.

Regulation of thyroid metabolism can be broken down into the following five steps:

1. The hypothalamus (a pea-sized gland in the brain) monitors the levels of thyroid hormone in the body and produces thyrotropin releasing hormone (TRH).
2. TRH acts on the anterior pituitary (directly below the hypothalamus, but outside of the blood-brain barrier) to produce thyrotropin, a.k.a. thyroid stimulating hormone (TSH).
3. TSH acts on the thyroid gland, which produces thyroxine (T4) and triiodothyronine (T3), the primary circulating thyroid hormones. The thyroid produces T4 in significantly greater quantities (in a ratio of 17:1) than T3, which is approximately 5x more biologically active than T4.
4. T4 is converted into the more active T3 by the deiodinase system (D1, D2, D3) in multiple tissues and organs, but especially in the liver, gut, skeletal muscle, brain and the thyroid gland itself. D3 converts T3 into an inactive form of thyroid hormone in the liver.
5. Transport proteins produced by the liver – thyroid binding globulin (TBG), transthretin and albumin – carry T4 and T3 to the tissues, where they are cleaved from their protein-carriers to become free T4 and free T3 and bind to thyroid hormone receptors (THRs) and exert their metabolic effect.

http://chriskresser.com/low-t3-syndrome-i-its-not-about-the-thyroid
A long series which answer "everything you ever wanted to know about your thyroid."