Radiology cases, Hyperparathyroidism.

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Hyperparathyroidism.

Calcium is essential to many cellular processes, particularly mechanisms for influencing membrane permeability as in the nerves, cardiac muscle or skeletal muscle. Availability of ionised calcium is thus essential to the organism and is closely regulated. This process is associated with the production of hormone from the parathyroid glands which increases bone turnover. Calcium, as hydroxyapatite, is an essential component of bone matrix and the largest store of that ion. When calcium is mobilised. The metaphyses of bones are particularly affected, and any additional area where there is a greater bone turnover than other sites can show increased bone resorption. The isotope scan below indicates the usual active areas. Put simply, the effect of the parathyroid hormone is to maintain or elevate the serum free calcium. Calcium will be mobilised from whatever source is possible, increasing resorption from bowel and renal tubules. Decreased resorption of phosphate, with a potential reduction in the calcium phosphate solubility product, will also help mobilise Calcium from the bones. The presence of Vitamin D. is necessary for the hormone to have its full effect upon the bone (and gut). Thus the patterns of primary and secondary hyperparathyroidism will be different. In the primary condition there is absorption of the tufts of the terminal phalanges in the hands and feet and subperiosteal bone resorption with particular effect at the level of the bone metaphysis.

Cysts can form anywhere is primary hyperparathyroidism and these are often well defined lytic lesions within the bone which resemble simple bone cysts. Under the microscope they contain a mixture of spindle cells and fibroblasts. If the primary hyperparathyroidism is treated these bone cysts will often resolve, leaving an area of sclerosis. The 'brown tumour' is characterised by the haemorrhage into the cyst in which case the cysts can often be permanent as a lytic area. Close examination of the hand radiographs is the most helpful in hyperparathyroidism and in which case the phalanges are seen to be asymmetrical with subcortical bone resorption on their radial side. A minute examination of the tufts will usually reveal that the continuous line of cortical bone which delinates the tuft is interrupted and the appearance resembles a lace border.

In hyperparathyroidism, the cystic areas of the brown tumours may not show-up with high diagnostic contrast on an isottope bopne scan, whcih selects for areas of increased bone turnover. It is worth remembering that for the unossified matrix in the cystic areas to take up Techetium 99m, the isotope has to get into the lesions, which therefore may not show as highly active areas.

In the kidney, parathyroid hormone, parathyrin, increases renal excretion of phosphate and decreases calcium excretion. The process is aided by Vitamin D. Increased filtration, despite increased resorption, of calcium will lead to the higher urine concentrations exceeding the calcium phosphate solubility product with the development of renal calculi and nephrocalcinosis. Renal failure may have paradoxical effects on calcium metabolism. The failure of hydroxylation of Vitamin D in the diseased renal cortex may ameliorate some of the effect of the metabolic acidosis. The serum concentration of magnesium in renal failure may alter the clinical expression of changes in calcium metabolism.