Hypothesis: the case for quaternary hyperparathyroidism.

We report the case of a young woman with hyperparathyroidism due to a large parathyroid adenoma associated with severe vitamin D deficiency. The case is noteworthy for the size of the parathyroid adenoma and for the young age at presentation, and is more typical of the presentation of hyperparathyroidism seen in developing countries where the prevalence of vitamin D deficiency is high. Vitamin D is known to have a suppressive effect on parathyroid cell proliferation and parathyroid hormone synthesis. Vitamin D deficiency may result in a compensatory increase in the secretion of parathyroid hormone (secondary hyperparathyroidism) which involves hyperplasia of all four parathyroid glands. Secondary hyperparathyroidism can become autonomous and this has been termed tertiary hyperparathyroidism, the underlying pathology of which has been variably described in the literature as adenoma formation or four gland hyperplasia. The pathogenesis of parathyroid adenoma formation in vitamin D deficiency remains unclear. It is possible that a proportion of cases represent the coincidence of primary hyperparathyroidism in patients with vitamin D deficiency. Alternatively, we hypothesise that autonomous four gland hyperplasia or tertiary hyperparathyroidism may progress to adenoma formation and that this should be termed 'quaternary hyperparathyroidism'.