Diagnostic ultrasound ( PDFDrive )

CHAPTER 20 The Parathyroid Glands 733
thyroid gland. Minimal migration occurs during fetal development,
and the superior parathyroids usually remain associated
with the posterior aspect of the middle to upper portion of the
thyroid gland. he majority of superior parathyroid glands (>80%)
are found at autopsy within a 2-cm area located just superior to
the crossing of the recurrent laryngeal nerve and the inferior
thyroid artery. 4
he inferior parathyroid glands arise from the paired third
branchial pouches, along with the thymus. 2 During fetal development,
these “parathymus glands” migrate caudally along with
the thymus in a more anterior plane than their superior counterparts,
bypassing the superior glands to become the inferior
parathyroid glands. 3 Because of their greater caudal migration,
the inferior parathyroid glands are more variable in location
than the superior glands and can be found anywhere from the
angle of the mandible to the pericardium. he majority of inferior
parathyroid glands (>60%) come to rest at or just inferior to the
posterior aspect of the lower pole of the thyroid 4 (Fig. 20.1).
A signiicant percentage of parathyroid glands lie in relatively
or frankly ectopic locations in the neck or mediastinum. Symmetry
to ixed landmarks occurs in 70% to 80%, so side-to-side
comparisons can oten be made. 3,4 he ectopic superior parathyroid
gland usually lies posterior to the esophagus or in the
FIG. 20.1 Location of Parathyroid Glands. Frequency of the location
of normal superior and inferior parathyroid glands. Anatomic drawing
from 527 autopsies. T, Thymus. (Modiied from Gilmour JR. The gross
anatomy of the parathyroid glands. J Pathol 1938;46:133-148. 1 )
tracheoesophageal groove, in the retropharyngeal space, or has
continued its descent from the posterior neck into the posterosuperior
mediastinum. 5,6 Superior glands are less oten found
higher in the neck, near the superior extent of the thyroid, or,
in rare cases, surrounded by thyroid tissue within the thyroid
capsule. 4 he inferior parathyroid gland is more frequently ectopic
than its superior counterpart. 4,6 About 25% of the inferior glands
fail to completely dissociate from the thymus and continue to
migrate in an anterocaudal direction and are found in the low
neck along the thyrothymic ligament or embedded within or
adjacent to the thymus in the low neck and anterosuperior
mediastinum. Less common ectopic positions of the inferior
parathyroid glands include an undescended position high in the
neck anterior to the carotid bifurcation associated with a remnant
of thymus, and lower in the neck along or within the carotid
sheath. 7 In other rare cases, ectopic glands have also been reported
in the mediastinum posterior to the esophagus or carina, in the
aortopulmonic window, within the pericardium, or even far
laterally within the posterior triangle of the neck.
Most adults have four parathyroid glands, two superior and
two inferior, each measuring about 5 × 3 × 1 mm and weighing
on average 35 to 40 mg (range, 10-78 mg). 3,8 Supernumerary
glands (>4) may be present and result from the separation of
parathyroid anlage when the glands pull away from the pouch
structures during the embryologic branchial complex phase. 9,10
hese supernumerary glands are oten associated with the thymus
in the anterior mediastinum, suggesting a relationship in their
development with the inferior parathyroid glands. 11 Supernumerary
glands have been reported in 13% of the population at autopsy
studies 3,4 ; however, many of these are small, rudimentary or split
glands. “Proper” supernumerary glands (>5 mg and located well
away from the other four glands) are found in 5% of cases. he
presence of fewer than four parathyroid glands is rare clinically
but has been reported in 3% at autopsy.
Normal parathyroid glands vary from a yellow to a red-brown
color, depending on the degree of vascularity and the relative
content of yellow parenchymal fat and chief cells. 8 he chief cells
are the primary source for the production of parathyroid
hormone (PTH, parathormone). he percentage of glandular
fat typically increases with age or with disuse atrophy. Hyperfunctioning
glands resulting from adenomas or hyperplasia
contain relatively little fat and are vascular, thus more reddish.
he glands are generally oval or bean shaped but may be spherical,
lobular, elongated, or lattened. Although normal parathyroid
glands are occasionally seen with high-frequency ultrasound, 12,13
typically they are not visualized, likely because of their small
size, deep location, and poor conspicuity related to increased
glandular fat. Eutopic parathyroid glands typically derive their
major blood supply from branches of the inferior thyroid artery,
with a lesser and variable contribution to the superior glands
from the superior thyroid artery. 3,7
PRIMARY HYPERPARATHYROIDISM
Prevalence
Primary hyperparathyroidism is a common endocrine disease,
with prevalence in the United States of 1 to 2 per 1000