Introduction
The thyroid gland is a butterfly-shaped endocrine gland located in the anterior neck, in front of the trachea. It plays a vital role in regulating metabolism through hormone production.
Shape and Location
The gland is H-shaped, with two lobes connected by a central isthmus. It lies against the C5 to T1 vertebrae.
• Each lobe extends from the middle of the thyroid cartilage to the fifth tracheal ring.
• The isthmus extends across the second to fourth tracheal rings.
Weight and Measurement
• Weight: Approximately 25 grams in adults.
• Overall Size: About 5 × 2.5 × 2.5 cm for each lobe.
• Isthmus: Around 1.2 × 1.2 cm.
Capsule of the Thyroid Gland
• True Capsule: A fibrous capsule made by the condensation of connective tissue. It is cosely adhered to the gland. Beneath it lies a dense capillary plexus.
• False Capsule: Derived from the pretracheal layer of deep cervical fascia. It is thinner at the posterior border of each lobe.
Parts and External Features
The thyroid consists of two lobes and a connecting isthmus:
• Lobes: Each has an apex (pointing upward), a base, three surfaces (medial, lateral, and posterolateral), and two borders (anterior and posterior).
• Isthmus: Connects the lower part of both lobes and has four surfaces — anterior, posterior, superior, and inferior.
Blood Supply
• Superior thyroid artery: Branch of the external carotid artery.
• Inferior thyroid artery: Branch of the thyrocervical trunk (from the subclavian artery).
• Accessory thyroid artery (if present): A branch from the esophageal arteries.
Lymphatic Drainage
Lymph from the thyroid gland drains into the deep cervical lymph nodes, particularly those surrounding the internal jugular vein.
Nerve Supply
Innervation is primarily sympathetic, from the:
• Superior cervical ganglion
• Middle cervical ganglion
• Inferior cervical ganglion
These nerves are vasomotor, regulating blood flow but not hormone secretion (which is controlled hormonally, not neurally).
Clinical Anatomy
• Hyperthyroidism: Overactivity of the gland leading to excessive thyroid hormone production.
• Hypothyroidism: Underactivity resulting in reduced hormone levels.
• Goitre: An enlargement of the thyroid gland, often due to iodine deficiency or other metabolic imbalances.