Parathyroid
glands
Anatomy. The parathyroid glands, four in
number, are small, oval in shape, commonly about 0.5 cm in size, soft, mobile, yellowish brown in colour and arranged
in pairs — most often closely applied to the thyroid gland, either within or
closely applied to its capsule. The upper pair is more constant in position than
the lower: 80 per cent are found on the posterolateral aspect of the thyroid,
immediately above the termination of the inferior thyroid artery, close to the
cricothyroid articulation. Most of the remaining 20 per cent are posterolateral
to the upper pole of the thyroid lobe. The lower pair is more variable in
position: 40 per cent are found at the lower pole of the thyroid and 40 per cent
are within the thymic tongue (Fig. 45.1). The remaining 20 per cent are variable
in site, most often some distance lateral to the thyroid, and less often in the
mediastinal thymus a few centimeters below the sternal notch or, very
occasionally, ectopicallv situated near the carotid sheath, sometimes as high as
the carotid bifurcation. On rare occasions, a parathyroid, usually the upper
gland, may be retropharyngeal, retro-oesophageal or actually within the thyroid
substance and, in 1—2 per cent of individuals, there is one or more
supernumerary glands usually associated with a lobule of thymic tissue. Each
gland has a delicate capsule and is supplied by a single leash of blood vessels
clearly seen running in the subcapsular plane (Fig.
45.2). Very often,
parathyroid glands are associated with or embedded within a pad of fat, which
gives a useful clue to identification.
Histology.
The stroma consists of a rich sinusoidal capillary network with islands of
secretory cells interspersed with fat cells. The glandular cells are of two
types. The ‘chief’ or ‘principal’ cells are small with vesicular nuclei
and poorly staining cytoplasm. ‘Water-clear’ cells, derived from the chief
cells, are found in hyperplastic and neoplastic glands. The ‘oxyphil’ cells
are less numerous and larger, with granular cytoplasm and deeply staining
nuclei.
Physiology.
The chief cells of the parathyroids produce parathormone, the hormone being
released directly into the bloodstream. The circulating level of parathormone
can be measured by radioimmunoassay, which is sufficiently reliable to
distinguish between high and low levels. Facilities for obtaining the estimation
are widely available.
Parathormone:
•
stimulates osteoclastic activity, thereby increasing bone resorption by mobilizing
calcium and phosphate;
•
increases the reabsorption of calcium by the renal tubules, thus reducing
the urinary excretion of calcium;
•
augments the absorption of calcium from the gut;
•
reduces the renal tubular reabsorption of phosphate, thus promoting
phosphaturia.
Parathyroid
hormone is an 84 amino acid peptide which has a short half-life before
degradation into amino-terminal and carboxy-terminal
Calcitonin
(Copp)
is secreted by the parafollicular cells of the thyroid (thyrocalcitonin). It
lowers the serum calcium and affects calcium storage in bones; quite the
opposite action of parathormone.
Parathyroid hormone-related protein (PTH-rP)
is
a hypercalcaemic