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."

Dr. Davis on T3 Supplementation Rx

T3 for accelerating weight loss  (excerpts follow)

Thyroid health, in general, is extremely important for weight control, since even subtle low thyroid hormone levels can result in weight gain. The first step in achieving thyroid health is to be sure you are obtaining sufficient iodine. (See Iodine deficiency is real and Healthy people are the most iodine deficient) But, after iodine replacement has been undertaken, the next step is to consider your T3 status.

I've seen T3 ignite weight loss or boost someone out of a weight loss "plateau" many times.

Endocrinologists cringe at this notion of using T3. They claim that you will develop atrial fibrillation (an abnormal heart rhythm) and osteoporosis by doing this. I have yet to see this happen. 

T3 added to other weight reducing efforts, such as wheat elimination and exercise, accelerates the weight loss effect considerably. 5 lbs lost will likely be more like 8 to 10 lbs lost; 10 lbs lost will likely be more like 15 to 20 lbs, etc.

The tough part: Finding a prescriber for your T3.