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 fragments, with the amino-terminal fragment having biological activity. The amino-terminal fragment retains biological activity with a half-life of minutes and the carboxy-terminal fragment a half-life of hours. Available assays measure either the intact hormone, the amino- or carboxy-terminal or ‘mid-portion’ fragments.

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 factor with similar bioactivity to that of parathyroid hormone. Since its isolation from cancer cell lines and carcinoma of the breast, strong evidence has emerged that it is an important hormonal mediator of cancer-associated hypercalcaemia in patients with solid tumours. Plasma PTH-rP 1—86 concentrations may be measured by a two-site immunoradiometric assay.