Thyroid Gland Anatomy and Physiology

In-depth knowledge of thyroid gland embryology, anatomy, and physiology is crucial to a comprehensive understanding of operative thyroid disease and to the conduct of safe thyroid operations. The thyroid is the first endocrine gland to develop and arises in the pharynx before descending to its ultimate location on the anterior trachea. The fetal thyroid begins secreting hormone around 10–12 weeks gestation, and thyroid hormone is critical to fetal development. Thyroid hormone is produced in the follicular cells and requires iodine, which can only be acquired in the diet. Arterial supply and venous drainage of the thyroid are described in detail. Other major anatomic considerations include the recurrent laryngeal nerves and parathyroid glands; while injuries to these structures are feared complications of thyroid surgery, numerous research studies have shown that high-volume surgeons have lower rates of complications. Thyroid hormone synthesis is regulated by the hypothalamic-pituitary-thyroid axis, which is a negative feedback system. Thyroid hormone has important actions on many tissues, including in the cardiovascular, urologic, musculoskeletal, and nervous systems. Thyroid hormone can act via genomic action (within the nucleus of target cells) or via nongenomic action (at the plasma membrane, in the cytoplasm, or on the mitochondria). Graves’ disease, multinodular goiters related to iodine deficiency, and toxic thyroid nodules are all surgical conditions that arise from disordered thyroid physiology.