4 - The assessment of parathyroid function

Abstract

This chapter presents various methods of assessment of parathyroid function and the interpretation of that assessment in the light of current knowledge of parathyroid physiology and pathology. Parathyroid plays a central role in regulating the calcium concentration of extracellular fluid. Parathyroid hormone (PTH) acts primarily to conserve body calcium. This is achieved both by restriction of calcium excretion and by increased input of calcium into extracellular fluid. However, in addition to its stimulation of renal tubular reabsorption of calcium and bone mineral resorption, parathyroid hormone also interacts with the vitamin D endocrine system by increasing the activity of 25-hydroxycholecalciferol 1α-hydroxylase. The increased output of 1,25-dihydroxycholecalciferol, which results from this, leads in turn to the stimulation of intestinal calcium absorption. Parathyroid function is most frequently assessed in patients with disorders of calcium metabolism or hypercalcaemia. There is no doubt that the advent of parathyroid hormone immunoassay has made an enormous impact both on diagnosis and on the understanding of the often complex mechanisms of these diseases. Recent surveys stress that the high incidence of hyperparathyroidism seen at present stems from the finding of mild hypercalcaemia in largely asymptomatic patients subject to biochemical screening. The attempt to reach a diagnosis in cases with the mildest degree of disease provides a challenge to the accuracy and precision of laboratory technology. It may be stated that the application of these alternative investigations in general reflects the lack of availability of reliable parathyroid hormone immunoassays.