The thyroid gland, a butterfly-shaped organ located in the neck, regulates the body’s metabolism and energy use. It produces and secretes thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3). To maintain a stable supply, the thyroid utilizes a unique internal storage system involving a gelatinous substance called colloid. Colloid fills the microscopic storage units of the thyroid gland and is fundamental to the gland’s efficient operation.
The Thyroid Follicle: Colloid’s Physical Location
The functional units of the thyroid gland are small, spherical structures known as thyroid follicles. These follicles are essentially tiny sacs. The wall of each follicle is formed by a single layer of specialized cells called follicular cells, or thyrocytes.
The center of this spherical structure is a fluid-filled space known as the follicular lumen. This lumen is the physical location where the thyroid colloid is stored. The follicular cells have an apical membrane that faces the colloid in the lumen and a basolateral membrane that faces the surrounding capillaries. This arrangement is crucial for the two-way traffic of materials: substances are taken from the blood, processed by the follicular cells, deposited into the colloid, and then reabsorbed and released back into the blood.
The Chemical Makeup of Colloid
The gelatinous nature of the colloid is due to its primary component, a large glycoprotein called thyroglobulin (Tg). Thyroglobulin is synthesized exclusively by the follicular cells and then secreted into the follicular lumen, where it accumulates to form the colloid. This large protein molecule provides the structural framework for thyroid hormone production and storage.
The thyroglobulin molecule contains multiple tyrosine residues, which are the sites where iodine atoms are incorporated to build the hormones. Iodine, which the follicular cells actively concentrate from the bloodstream, is oxidized and then bound to these tyrosine residues within the colloid. This process of iodination results in the formation of iodinated tyrosine precursors that remain attached to the thyroglobulin protein.
The colloid is a highly concentrated solution of iodinated thyroglobulin, which acts as the precursor molecule for the active hormones. Approximately 0.2 to 1% of the colloid’s weight is elemental iodine. About 70% of this iodine is found in the inactive precursor forms, monoiodotyrosine and diiodotyrosine, while the remaining 30% is incorporated into the T3 and T4 hormones, all still bound within the thyroglobulin molecule.
Colloid’s Central Role in Thyroid Hormone Storage and Release
The accumulation of iodinated thyroglobulin within the colloid provides the body with a substantial reserve of thyroid hormones. This storage capacity is unique among endocrine glands, allowing the thyroid to hold several weeks, or even months, worth of hormones in an inactive state. The colloid acts as a buffer against fluctuations in dietary iodine intake, ensuring a steady supply of T4 and T3.
When the body requires active thyroid hormones, the follicular cells retrieve the colloid from the lumen through endocytosis. The colloid is engulfed by the follicular cells and internalized in vesicles. These vesicles then fuse with lysosomes inside the cell, where digestive enzymes hydrolyze and break down the thyroglobulin protein.
The breakdown process cleaves the active T4 and T3 molecules from the thyroglobulin structure, releasing them as free hormones. These hormones then diffuse out of the follicular cell and enter the adjacent capillaries to be distributed throughout the bloodstream to regulate metabolism. The stored colloid is continually being turned over, with the follicular cells balancing the secretion of new thyroglobulin into the lumen with the reabsorption and breakdown of the iodinated protein for hormone release.