The thyroid gland, a butterfly-shaped organ located at the base of the neck, functions as an endocrine gland, producing and releasing hormones directly into the bloodstream. This gland is composed of numerous microscopic structures known as thyroid follicles, the fundamental units responsible for its activity. Each follicle is a spherical sac lined by a single layer of specialized cells, and its central cavity, called the lumen, holds a unique substance essential for hormone production.
Thyroglobulin
The material stored within the lumen of the thyroid follicle is a viscous substance known as colloid. This colloid is predominantly made up of a large glycoprotein called thyroglobulin, the direct precursor to thyroid hormones. Thyroglobulin is synthesized by the follicular cells lining the follicle and then secreted into the lumen, where it accumulates. This protein molecule features numerous tyrosine amino acid residues that serve as attachment points for iodine, a trace element obtained from the diet.
Thyroid Hormone Formation Within the Follicle
Thyroid hormone synthesis begins with the active transport of iodide, the ionic form of iodine, from the bloodstream into the follicular cells. Once inside the cell, iodide is transported into the follicular lumen. Here, an enzyme called thyroid peroxidase oxidizes the iodide, converting it into an active form of iodine, which is then incorporated onto the tyrosine residues of the thyroglobulin molecule. This process, known as organification, results in the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT) units attached to the thyroglobulin backbone.
Subsequently, two iodinated tyrosine residues couple together within the thyroglobulin molecule. The coupling of one MIT and one DIT forms triiodothyronine (T3), while the coupling of two DIT units forms thyroxine (T4). These newly formed thyroid hormones, T3 and T4, remain bound to the thyroglobulin molecule within the colloid, stored until the body signals a need for them. This storage mechanism allows the thyroid gland to maintain a reserve of hormones.
Release and Action of Thyroid Hormones
When the body requires thyroid hormones, the follicular cells reabsorb portions of the iodinated thyroglobulin from the lumen through endocytosis. Once inside the follicular cells, these vesicles containing thyroglobulin merge with lysosomes. Lysosomal enzymes then cleave the T3 and T4 molecules from the thyroglobulin backbone. The free T3 and T4 are then released from the follicular cells and diffuse into the surrounding capillaries, entering the bloodstream.
Thyroid hormones circulate throughout the body, primarily bound to transport proteins. T3 and T4 regulate the body’s metabolism, influencing nearly every cell and tissue. They are involved in maintaining body temperature, controlling heart rate, and supporting normal growth and development. Their release from the stored thyroglobulin ensures a continuous supply to meet the body’s metabolic demands.