Thyroid Ultrasound > Introduction > Physiology


The thyroid gland synthesizes, stores, & secretes the thyroid hormones, principally thyroxine (T4) and to a lesser extent triiodothyronine (T3). Iodine trapped by Na-I cotransporter in cell membrane and transported to apical membrane (via transporter). Thyroglobulin is produced in stored in follicle. Thyroid peroxidase (TPO) ionizes iodine and links it to thyroglobulin covalently, a process called organification, leading to monoiodotyrosine, diodotyrosine, etc. Iodine is trapped, oxidized and covalently bound to tyrosines in thyroglobulin, a process which is vital in thyroid hormone production. Thyroid gland function is controlled by hypothalamic TRH and pituitary TSH which are in turn regulated by the negative feedback of thyroid hormones. 

Thyroglobulin levels are used to determine whether residual thyroid tissue is present in patients who have had thyroid ablation with radiation or surgery. 

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