Figure 45-1. Structures of the thyroid hormones thyroxine (T4) and tri-
iodothyronine (T3). Also shown are the intermediates monoiodotyrosine (MIT)
and diiodotyrosine (DIT) that are also formed on thyroglobulin.
at either one or two sites, and then these residues are coupled to generate either
T3 or T4 residues within thyroglobulin. The iodinated thyroglobulin is then
taken up from the extracellular matrix into the cytoplasm of the thyroid cell
where lysosomal proteases cleave T3 and T4 from thyroglobulin. The hor-
mones are then carried in the blood bound primarily to thyroid-binding glob-
ulin. T4 is converted to T3 in the liver and to a lesser extent other tissues,
accounting for 80 percent of circulating T3.
Thyroid hormones stimulate protein synthesis in most cells of the body.
They also stimulate oxygen consumption by increasing the levels of the Na+,
K+-ATPase ion transporter. The generation of plasma membrane Na+ and
K+gradients by the Na+, K+-ATPase is a major consumer of cellular adenosine
triphosphate (ATP), leading to stimulation of ATP synthesis in the mitochondria
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