The thyroid gland is important for control of metabolism. Thyroid malignancies are uncommon but due to the unique properties of the thyroid gland treatment is distinct.
Structure
The thyroid consists of two lobes and a connecting isthmus. The right and left lobes of the thyroid constitute most of the gland, and are united centrally and anteriorly by the isthmus. The gland is surrounded by a fibrous capsule which gives off septa into the substance of the gland. This capsule is firmly bound to the cricoid cartilage and the upper tracheal rings.
Function
The thyroid gland produces thyroxine, a hormone which regulates the metabolic activity of cells. Other cells produce calcitonin, important in calcium and phosphate homeostasis.
Relations
The gland lies anterior to vertebral levels C5 – T1.
The gland lies deep to the sternothyroid and sternohyoid strap muscles of the neck. The omohyoid coves the superior lateral parts; the sternocleidomastoid may overlap the inferolateral parts.
It extends from C5 to T1. The isthmus lies anterior to the trachea, usually abutting the 2nd and 3rd tracheal rings.
Macroscopic Features
The thyroid gland is notable for rich vasculature giving it a reddish-brown hue. The surface appears lobulated.
Microscopic Features
The thyroid gland is covered by a thin capsule. The gland is filled with follicles, which consist of central colloid and a surrounding epithelium (follicular cells). The follicular cells are involved in the production of thyroxine. Between the follicles is a small amount of vascular stroma, containing the clear cells of the thyroid. These cells are involved in the production of calcitonin.
Variations
A third lobe of the thyroid (the pyramidal lobe) is occasionally present. It may arise from the isthmus or one of the major lobes, and passes superiorly in the neck.
Neurovascular Supply
Arterial Supply
The thyroid receives a relatively large superior and inferior artery from each side. The inferior thyroid artery arises from the thyrocervical trunk, a short stub itself arising from the subclavian artery. The superior thyroid artery is the second branch of the external carotid artery, which descends inferiorly to enter the superior substance of the gland.
Occasionally a thyroid ima artery may arise from the brachiocephalic artery, passing superiorly to enter the isthmus or inferior part of one of the lobes.
Venous Drainage
A pretracheal venous plexus posterior to the gland gives rise to three veins on each side. The superior thryoid vein accompanies the artery, passing superiorly and laterally to empty into the internal jugular vein. The middle thyroid vein passes laterally and also reaches the internal jugular. The inferior veins do not accompany the inferior thyroid artery, instead passing inferior to empty into the right or left brachiocephalic vein.
Lymphatics
Lymph from the thyroid gland may drain to numerous locations, including:
- Pretracheal and paratracheal nodes (station VI)
- Inferior deep cervical nodes (station IV)
- Anterior mediastinal nodes
Innervation
The thyroid gland receives sympathetic input from the three cervical ganglia. General sensory and parasympathetic supply is not described.
Routes of Cancer Spread
Local Invasion
Local growth of a malignancy can lead to compression of the trachea and/or oesophagus, giving rise to the expected symptoms. The recurrent laryngeal nerve may be involved, leading to hoarseness or loss of voice. Tumours can also ulcerate through the skin although this is less common due to the overlying fascia.
Lymphatic Spread
Thyroid cancer may spread to numerous lymphatics in the neck area, making surgical excision more difficult.
Haemotogenous Spread
Haemotogenous spread may occur. Cure is still possible with radioactive iodine-131 treatments.
