Thyroid and Parathyroid

patient. The following year, Edward Fox demonstrated that oral therapy in the form of
“half a sheep's thyroid, lightly fried and taken with currant jelly once a week” was
. equally effective
Few of Billroth's patients developed myxedema, but William Halsted suggested that
this was because of a difference in operative technique. Kocher was extremely neat
and precise, operating slowly in a bloodless field. He removed all the thyroid, and his
patients developed myxedema but rarely suffered laryngeal nerve damage or
postoperative tetany. Billroth, however, worked rapidly and with less concern for
hemorrhage. He often removed the parathyroid glands but left more thyroid tissue and
therefore encountered postoperative hypoparathyroidism but rarely myxedema. In
1909 Kocher received the Nobel Prize for medicine in recognition “for his works on
” . the physiology, pathology, and surgery of the thyroid gland
EMBRYOLOGY
A clear understanding of the developmental embryology and anatomy of the thyroid
gland is essential for the clinician performing a thorough physical examination of the
gland and aids in evaluating diagnostic images. Knowledge of possible developmental
anomalies and the thyroid gland's relationship to the parathyroid glands and other
. neck structures is vital in performing safe and effective thyroid operations
The thyroid gland originates from the base of the tongue in the region of the foramen
cecum. Embryologically, it is an offshoot of the primitive alimentary tract. The
endoderm cells in the midline of the floor of the pharyngeal anlage thicken and form a
median thyroid anlage, which migrates caudally into the neck (Fig. 36-2). The anlage
descends along a tract that runs anterior to the structures that form the hyoid bone and
the larynx; it is composed of epithelial cells that provide the follicular cells of the
thyroid. As it descends, it is joined laterally by a pair of components originating from
the ultimobranchial bodies of the fourth and fifth branchial pouches. These lateral
components supply the C cells of the thyroid, which secrete calcitonin. When the C
cells become neoplastic, the result is medullary carcinoma of the thyroid. An
understanding of this anatomy explains why medullary carcinoma usually is located
in the upper poles of the thyroid and virtually never in the isthmus or pyramidal lobe.
The thyroid gland forms follicles by the end of the tenth week of gestation and
. concentrates iodine and produces colloid by the end of the twelfth week
ANOMALIES
Rarely, the thyroid gland, whole or in part, descends more caudally. This results in
thyroid tissue located in the superior mediastinum behind the sternum, adjacent to the
aortic arch or between the aorta and the pulmonary trunk, within the upper portion of
the pericardium, or in the interventricular septum. The following types of anomaly
. can be encountered
Pyramidal Lobe
The migratory tract of the developing thyroid gland is known as the thyroglossal tract
or duct. Normally the duct atrophies, although it may remain as a fibrous band. In
about 80 percent of people, the distal end that connects to the thyroid persists as a
pyramidal lobe projecting up from the isthmus, lying just to the left of the midline
(Fig. 36-3). In the normal individual the pyramidal lobe is not palpable, but in
disorders resulting in thyroid hypertrophy (e.g., Graves' disease, diffuse nodular