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Primary hyperparathyroidism (PHPT) is one of the most common endocrine diseases. Its clinical presentation has dramatically changed in the last 40 years, and now the disease typically affects elderly women and is characterized by mild hypercalcemia and few traditional classic (bone and kidney) manifestations. The change in clinical presentation was largely caused by development of automated serum calcium measurement in the early 1970s that made possible the introduction of serum calcium determination in the routine biochemical screening and the identification of a large number of" asymptomatic" patients. This led also to a 5-fold increase in the apparent incidence of PHPT for the identification of patients who were never diagnosed before (catch-up effect). From the most recent and accurate study, a 21/1000 PHPT prevalence was found in women aged 55-75 years, which is equivalent to 3/1000 prevalence in the general population. Epidemiological studies performed in Rochester, Minnesota, have shown an apparent decline in the annual incidence from 75 to approximately 20/100,000 in the last decade. Most of this apparent decline in the incidence of PHPT is explained by the decline of the" catch-up effect," although a number of factors that might result in changes in PHPT incidence have been identified. Vitamin D deficiency is particularly common in people living in Southern European countries, and this may contribute to underestimating the prevalence of PHPT, because total serum calcium may be normal in these patients. On the other hand vitamin D deficiency may be associated with more severe clinical expression of the disease …