Primary Hyperparathyroidism Superimposed on a Pre-existing Familial Hypocalciuric Hypercalcemia Diagnosis

Familial hypocalciuric hypercalcemia is a rare clinical condition of persistently elevated serum calcium and reduced urinary calcium levels with an autosomal dominance inheritance pattern to the three out of four large types of this condition known. This rare condition goes largely undiagnosed as patients are largely asymptomatic and where symptoms are present, other causes of hypercalcemia are considered first. Hyperparathyroidism, super-imposing on FHH, is an even rarer occurrence. We present the case of an adult male with an initial provisional assessment of FHH, which was later confirmed with a genetic study. He went on to develop hyperparathyroidism (with evident enlarged parathyroid glands on Sestamibi parathyroid scan done, and an eventual histologic diagnosis of parathyroid adenoma after surgery). It remains to be established if this is an incidental occurrence or if there is a causal relationship between FHH and an onward development of parathyroid hypertrophy or adenoma(ta).