The thyroid gland is a butterfly-shaped, highly vascular endocrine organ located in the anterior neck. It is essential for regulating metabolism, growth, and development by secreting thyroid hormones.
1. Gross Anatomy
- Location and Structure:
- Positioned in the anterior neck, extending from the C5 to T1 vertebrae.
- Consists of two lobes (right and left) connected by a central isthmus.
- Each lobe is roughly 4-6 cm in length and 1.5-2 cm in width.
- Occasionally, a pyramidal lobe, an embryological remnant of the thyroglossal duct, extends upward from the isthmus.
- Capsules:
- Enclosed by a true fibrous capsule and a surrounding false capsule derived from the pretracheal fascia.
- The gland is anchored to the trachea by connective tissue, causing it to move during swallowing.
2. Microscopic Anatomy
- Thyroid Follicles:
- The functional units of the thyroid are spherical follicles.
- Each follicle is lined with a single layer of cuboidal to columnar epithelial cells known as follicular cells.
- The interior of the follicles is filled with colloid, a protein-rich substance that stores thyroglobulin, the precursor for thyroid hormones.
- Cell Types:
- Follicular Cells: Involved in the synthesis of thyroid hormones (T3 and T4).
- Parafollicular Cells (C Cells): Located between follicles, these cells secrete calcitonin, a hormone that helps regulate calcium levels.
3. Vascular Supply
- Arterial Supply:
- Superior Thyroid Artery: Originates from the external carotid artery and supplies the upper parts of the gland.
- Inferior Thyroid Artery: Arises from the thyrocervical trunk, supplying the lower portions.
- Thyroid Ima Artery (if present): A variable artery that may arise directly from the aorta or brachiocephalic trunk.
- Venous Drainage:
- Superior Thyroid Vein: Drains into the internal jugular vein.
- Middle Thyroid Vein: Also drains into the internal jugular vein.
- Inferior Thyroid Vein: Drains into the brachiocephalic veins.
- Lymphatic Drainage:
- Drains to deep cervical and paratracheal lymph nodes, including prelaryngeal and pretracheal nodes.
4. Nerve Supply
- Sympathetic Innervation: Originates from the cervical sympathetic ganglia, primarily affecting blood flow rather than hormone secretion.
- Recurrent Laryngeal Nerve: Runs near the inferior thyroid artery, supplying motor innervation to the larynx and potentially at risk during thyroid surgery.
5. Embryological Development
- Thyroid Gland Development:
- Originates from the endodermal tissue at the base of the tongue, specifically from the foramen cecum.
- Migrates downwards along the thyroglossal duct to reach its final position in the neck.
- The thyroglossal duct normally regresses, but remnants can form cysts.
- Parafollicular Cell Development:
- Derived from the neural crest cells associated with the ultimobranchial body, which fuses with the developing thyroid.
6. Physiological Function
- Thyroid Hormone Production:
- Thyroxine (T4) and Triiodothyronine (T3): Regulate metabolic processes throughout the body.
- Hormones are synthesized from thyroglobulin, which is iodinated within the follicles.
- Calcitonin: Produced by parafollicular cells, helps regulate blood calcium levels by inhibiting osteoclast activity.
7. Clinical Correlations
- Goiter: Enlargement of the thyroid gland, which can compress the trachea, esophagus, or nearby structures.
- Hyperthyroidism: Excess hormone production, leading to symptoms like weight loss, heat intolerance, and tachycardia.
- Hypothyroidism: Insufficient hormone production, causing fatigue, weight gain, and cold intolerance.
- Thyroid Cancer: Can arise from follicular cells or parafollicular cells (medullary thyroid cancer).
- Thyroglossal Duct Cysts: Midline neck masses resulting from remnants of the thyroglossal duct.
