Thyroid anatomy and the laryngeal nerves: fascia, blood supply, parathyroids and what injury does

The gland and its fascial envelope

The thyroid is a butterfly of two lobes joined by an isthmus across the second, third and fourth tracheal rings. Each lobe reaches from the oblique line of the thyroid cartilage to about the fifth or sixth tracheal ring, and in roughly half of people a pyramidal lobe rises from the isthmus, usually to the left of the midline, as a remnant of the thyroglossal duct along which the gland descended from the foramen caecum of the tongue. The gland has two capsules. The true capsule is its own fibrous coat, beneath which the arterial branches and venous plexus lie, so capsular dissection keeps the surgeon close to the gland and away from the nerves. The false capsule is the pretracheal layer of the deep cervical fascia, thickened posteriorly into the suspensory ligament of Berry, which binds each lobe to the cricoid cartilage and the first two tracheal rings. Because the larynx rises on swallowing and the gland is tied to it, a thyroid swelling rises too: the bedside sign that a neck lump is thyroid. A thyroglossal cyst also rises when the tongue is protruded, because the duct runs through the tongue base. One strap muscle matters here: sternothyroid attaches to the oblique line above the gland and stops it growing upward, so an enlarging goitre tends to go downward behind the sternum.

• In front: sternothyroid, with sternohyoid and the superior belly of omohyoid superficial to it and sternocleidomastoid overlapping laterally; the gland lies in the muscular triangle of the neck. The strap muscles are supplied by the ansa cervicalis (C1–C3) entering low down, so if they must be divided they are cut high.
• Medially: the larynx and trachea, the pharynx and oesophagus behind them, the recurrent laryngeal nerve in the groove between trachea and oesophagus, and the external laryngeal nerve on the inferior constrictor.
• Behind and laterally: the carotid sheath with the common carotid artery, internal jugular vein and vagus nerve, and the sympathetic trunk behind it on prevertebral fascia. A large goitre pushes the sheath laterally; a malignant one invades it.
• Behind: the parathyroid glands and the terminal branches of the inferior thyroid artery, between the true and false capsules.

Blood supply, venous drainage and lymphatics

The recurrent laryngeal nerves

Both recurrent laryngeal nerves are branches of the vagus, and both are recurrent because an artery of the embryonic aortic arches hooks them. The right nerve leaves the vagus in front of the subclavian artery, loops beneath it and ascends obliquely to reach the tracheo-oesophageal groove only in its upper part, so it crosses the lower pole at an angle and is the more variable. The left nerve leaves the vagus in the thorax, hooks beneath the arch of the aorta lateral to the ligamentum arteriosum and ascends vertically in the groove the whole way, so it is more constant but at the mercy of mediastinal disease: a left cord palsy can be the first sign of an aortic arch aneurysm or a bronchial carcinoma. Each nerve passes in front of, behind or between the branches of the inferior thyroid artery, runs within a few millimetres of Berry's ligament, where it is most often injured, and enters the larynx behind the cricothyroid joint beneath the inferior constrictor. It supplies every intrinsic muscle of the larynx except cricothyroid, including the only abductor of the cords, posterior cricoarytenoid, and carries sensation from below the cords. In about one person in two hundred the right nerve is non-recurrent: when the right subclavian arises from the arch distal to the left subclavian and passes behind the oesophagus, nothing hooks the nerve, and it runs straight from the vagus to the larynx at the level of the thyroid, where it is easily mistaken for a branch of the inferior thyroid artery.

The external laryngeal nerve and Joll's triangle

The superior laryngeal nerve leaves the vagus below the skull base and divides at the level of the hyoid. Its internal branch is sensory: it pierces the thyrohyoid membrane with the superior laryngeal artery to supply the mucosa above the vocal cords and carries the cough reflex that protects the airway from a misdirected swallow. Its external branch is motor to one muscle, cricothyroid, which tilts the thyroid cartilage forward on the cricoid to tense and lengthen the cords. It descends on the inferior constrictor just medial to the superior thyroid artery and is in danger when the superior pole is taken. The space in which it is found is Joll's triangle: bounded laterally by the superior pole and the superior thyroid vessels, above by the attachment of the strap muscles to the hyoid and medially by the midline, with cricothyroid as its floor. In most people the nerve leaves the vessels a centimetre or more above the pole, but in about a fifth it crosses below the level of the pole within the fat of the pedicle, which is why the pole is mobilised downward and its vessels ligated individually on the capsule rather than clamped en masse.

The parathyroids

There are usually four parathyroid glands, each about the size of a lentil (around 40 mg) and a yellow-brown colour that distinguishes them from fat and from the darker thyroid. They lie on the back of the lobes between the true and false capsules, and their positions follow their embryology. The superior glands come from the fourth pharyngeal pouch and have the shorter journey, so they are constant: on the back of the lobe at about the level of the cricoid, a centimetre or so above the point where the inferior thyroid artery crosses the recurrent laryngeal nerve, and behind the plane of the nerve. The inferior glands come from the third pouch with the thymus and are dragged down with it, so they vary: usually near the lower pole in front of the plane of the nerve, but sometimes in the thyrothymic ligament, in the thymus in the superior mediastinum or, if descent failed, high in the neck beside the carotid. Both pairs are supplied chiefly by the inferior thyroid artery through end-arteries, which is why that artery is ligated on the capsule rather than at its trunk and why a parathyroid that has gone dusky is minced and autotransplanted into sternocleidomastoid. Parathyroid hormone raises serum calcium by mobilising bone, increasing renal reabsorption and activating vitamin D; the thyroid's own parafollicular C cells, which migrate in from the ultimobranchial body, secrete calcitonin, which matters less for calcium than as the marker of medullary carcinoma. Remove or devascularise the parathyroids and calcium falls over the first day or two: tingling around the mouth and in the fingers, Chvostek's sign on tapping the facial nerve, Trousseau's carpal spasm with an inflated cuff and, untreated, tetany and a long QT interval.

Haematoma

Bleeding beneath the strap muscles compresses the trachea and obstructs venous drainage of the larynx, so the airway closes from oedema as well as pressure. A bedside emergency: skin exposure, cut the sutures, open the skin, open the muscles, pack (SCOOP), then return to theatre.

Hypocalcaemia

Transient after total thyroidectomy in a sizeable minority, permanent in a small one. Check calcium within a day; treat symptoms with oral calcium and activated vitamin D, and intravenous calcium gluconate for tetany or a long QT.

Nerve palsy

Recurrent laryngeal, external laryngeal or both. Hoarseness, a weak cough or a voice that tires; stridor if bilateral. Document cord movement by laryngoscopy before and after surgery.

Thyroid storm and hypothyroidism

Storm is a crisis of thyroxine release in an inadequately prepared toxic patient (fever, tachycardia, delirium), treated with beta-blockade, antithyroid drugs, steroids and cooling. Hypothyroidism follows total thyroidectomy inevitably, so replacement is planned from the outset.