Figure 45-2. Biosynthesis of the thyroid hormones T3 and T4 in the thyroid
follicular cell and release into the bloodstream. Abbreviations are as follows:
TG, thyroglobulin; MIT, monoiodotyrosine; DIT, diiodotyrosine; T3, tri-
iodothyronine; T4, thyroxine.
thus directly increasing mitochondrial energy metabolism. By this means, thy-
roid hormones aid the conversion of food into energy and heat. In all of its
known actions, thyroid hormone exerts its effects by interaction with its recep-
tor in the cell nucleus and activation of transcription of the target genes.
Thyroid hormone receptors are members of a large nuclear receptor super-
family that includes receptors for steroid hormones, vitamin D3, and retinoic