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
Thyroglossal fistula
Thyroglossal flstula (Fig. 44.6a, b) is never congenital: it
follows infection
or inadequate removal of a thyroglossal cyst. Characteristically
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.
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.
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.