#Thyroid medication free#
This “packaging” prevents their free diffusion into body cells. The remaining 99 percent of circulating T3 and T4 is bound to specialized transport proteins called thyroxine-binding globulins (TBGs), to albumin, or to other plasma proteins. This free T3 and T4 can cross the lipid bilayer of cell membranes and be taken up by cells. In the bloodstream, less than one percent of the circulating T3 and T4 remains unbound. These hormones remain in the thyroid follicles until TSH stimulates the release of free T3 and T4 into the bloodstream. The iodide ions undergo oxidation (i.e., their negatively charged electrons are removed) and enzymes link the iodine to tyrosine to produce triiodothyronine (T3), a thyroid hormone with three iodines, or thyroxine (T4), a thyroid hormone with four iodines. As a result, the concentration of iodide ions “trapped” in the thyroid cells is many times higher than the concentration in the bloodstream. The following steps outline the hormone’s assembly: Binding of TSH to thyroid receptors causes the cells to actively transport iodide ions across their cell membrane from the bloodstream. T3 and T4 hormones are produced when iodine attaches to a glycoprotein called thyroglobulin.
Thyroid hormone production is dependent on the hormone’s essential component: iodine. Figure 9.18 The Thyroid Gland Synthesis and Release of Thyroid Hormones Each of the thyroid lobes are embedded with parathyroid glands. The thyroid is a butterfly-shaped organ located anterior to the trachea, just inferior to the larynx (see Figure 9.18).