Embryology

The thyroid gland develops from the median bud of the pharynx (the thyroglossal duct) which passes from the foramen caecum at the base of the tongue to the isthmus of the thyroid. The ultimobranchial body which arises from a diverticulum of the fourth pharyngeal pouch of each side amalgamates with the corresponding lateral lobe. Parafollicular cells (C-cells) are derived from the neural crest and reach the thyroid via the ultimobranchial body. Recently, consideration has been given to the possibility that some C-cells are of endodermal rather than neural crest origin. It is doubtful whether the branchial apparatus itself contributes to the thyroid follicular cells.

Surgical anatomy (Figs 44.1 and 44.2)

The normal gland weighs 20—25 g. The functioning unit is the lobule supplied by a single arteriole and consisting of 24—40 follicles which are lined by cuboidal epithelium. The resting follicle contains colloid in which thyroglobulin is stored. The arterial supply is rich, and extensive anastomoses occur between the main thyroid arteries and branches of tracheal and oesophageal arteries. There is an extensive lymphatic network within the gland. Although some lymph channels pass directly to the deep cervical nodes, the subcapsular plexus drains principally to the juxtathyroid nodes, i.e. pretracheal (Delphic)’ and paratracheal nodes, and nodes on the superior and inferior thyroid veins, and thence to the deep cervical and mediastinal group of nodes.

Ectopic thyroid and anomalies of the thyroglossal tract

Some residual thyroid tissue along the course of the thyrogbossal tract is not uncommon, and may be lingual, cervical or intrathoracic. Very rarely the whole gland is ectopic.

Lingual thyroid

This forms a rounded swelling at the back of the tongue at the foramen caecum (Figs 44.3 and 44.4) and it may represent the only thyroid tissue present. It may cause dysphagia, impairment of speech, respiratory obstruction or hemorrhage. It is best treated by full replacement with thyroxine when it should get smaller, but excision or ablation with radioiodine is sometimes necessary.

Median (thyroglossal) ectopic thyroid

This forms a swelling in the upper part of the neck (Fig. 44.4) and is usually mistaken for a thyroglossal cyst. Again, this may be the only normal thyroid tissue present.

Lateral aberrant thyroid

There is no evidence that aberrant thyroid tissue ever occurs in a lateral position (Willis). ‘Normal thyroid tissue’ found laterally, separate from the thyroid gland, must be considered and treated as a metastasis in a cervical lymph node from an occult thyroid carcinoma, almost invariably of papillary type. Struma ovarii is not ectopic thyroid tissue, but part of an ovarian teratoma. Very rarely, neoplastic change occurs or hyperthyroidism develops.

Thyroglossal cyst

This may be present in any part of the thyroglossal tract (Fig. 44.5). The common situations, in order of frequency, are beneath the hyoid, in the region of the thyroid cartilage, and above the hyoid bone. Such a cyst occupies the midline, except in the region of the thyroid cartilage, where the thyroglossal tract is pushed to one side, usually to the left. It is to be remembered that the swelling moves upwards on protrusion of the tongue as well as on swallowing because of the attachment of the tract to the foramen caecum.

A thyroglossal cyst should be excised because infection is inevitable, owing to the fact that the wall contains nodules of lymphatic tissue   which communicate by lymphatics with the lymph nodes of the neck. An infected cyst is often mistaken for an abscess and incised, which is one way in which a thyroglossal fistula arises.

Thyroglossal fistula

Thyroglossal flstula (Fig. 44.6a, b) is never congenital: it follows infection or inadequate removal of a thyroglossal cyst. Characteristically   the cutaneous opening of such a fistula is drawn upwards on protrusion of the tongue. A thyroglossal fistula is lined by columnar epithelium, discharges mucus, and is the seat of recurrent attacks of inflammation.

Treatment. Because the thyroglossal tract is so closely related to the body of the hyoid bone, this central part must be excised, together with the cyst or fistula, or recurrence is certain. When the thyroglossal tract can be traced upwards towards the foramen caecum, it must be excised with the central section of the body of the hyoid bone, and a central core of lingual muscle (Sistrunk’s operation).

Physiology. The hormones tri-iodothyronine (T3) and thyroxine (T4) (extracted by E.G. Kendall in 1916) are bound to thyroglobulin within the colloid. Synthesis within the thyroglobulin complex is controlled by several enzymes, in distinct steps:

trapping of inorganic iodide from the blood;

oxidation of iodide to iodine;

binding of iodine with tyrosine to form iodotyrosines;

coupling of mono-iodotyrosines and di-iodotyrosines to form 13 and T4

when hormones are required the complex is resorbed into the cell and thyroglobulin broken down; T3 and T4 are liberated and enter the blood where they are bound to serum proteins: albumin and thyroxine binding globulin (TBG) and prealbumin (TBPA). A small amount of hormone remains free in the serum in equilibrium with the protein-bound hormone and is biologically active. The metabolic effects of the thyroid hormones are due to unbound free 14 and 13 (0.03 per cent and 0.3 per cent of the total circulating hormones, respectively). 13 is quick acting (within a few hours) whereas 14 acts more slowly (4—14 days). 13 is the more important physiological hormone and is also produced in the periphery by conversion from 14.

Therapeutic notes: L-thyroxine (T4) is the official name; trade name Eltroxin; tablet size 0.1 mg and 0.05 mg. Tri-iodothyronine (T3), official name liothyronine; trade names Cynomel, Tertroxin; tablet size 20 .tg.

  Thyrocalcitonin

  See calcitonin, Chapter 45.

  The pituitary thyroid axis

Synthesis and liberation of thyroid hormones from the thyroid is controlled by thyroid-stimulating hormone (TSH) from the anterior pituitary. Secretion of TSH depends upon the level of circulating thyroid hormones and is modified in a classic negative feedback manner. In hyperthyroidism, where hormone levels in the blood are high, TSH production is suppressed whereas in hypothyroidism it is stimulated. Regulation of TSH secretion also results from the action of thyrotrophin-releasing hormone (TRH) produced in the hypothalamus.

Thyroid.stimulating antibodies

A family of IgG immunoglobulins binds with TSH receptor sites (TRAbs) and activate TSH receptors on the follicular cell membrane. They have a more protracted action than TSH (16—24 hours versus 1.5—3 hours) and are responsible for virtually all cases of thyrotoxicosis not due to autonomous toxic nodules. Serum concentrations are very low and not routinely measured.