Unraveling the Paradox of Vitamin D Status in Primary Hyperparathyroidism: An Incidental Finding or an Unexpected Consequence?

Suboptimal vitamin D status is commonly observed in primary hyperparathyroidism but is rarely considered in management decisions. The present study aimed to bring additional insights on vitamin D status in primary hyperparathyroidism patients, particularly those presenting with the normocalcemic phenotype. A retrospective study was conducted on 53 confirmed primary hyperparathyroidism patients, stratified into hypercalcemic and normocalcemic groups, hospitalized at the "Pius Brînzeu" Emergency Clinical Country Hospital in Timișoara, Romania. Patients presenting with the normocalcemic phenotype had similar target-organ involvement compared to their counterparts. In this subgroup, 25 hydroxyvitamin D showed an inverse correlation with serum calcium ( = 0.048), and regression analysis identified iPTH and 25OH vitamin D as significant predictors of calcium levels ( < 0.0001; R2 = 0.571). Adenoma volume showed a significant negative correlation with 25OH vitamin D levels ( = 0.021; r = -0.61) but was later found as insignificant after confounder analysis. Postoperative measurements of 25OH vitamin D levels confirmed increasing levels after parathyroidectomy. Our findings highlight a complex relationship between PTH and vitamin D in primary hyperparathyroidism, especially in the often-underdiagnosed normocalcemic phenotype. The inverse correlation between vitamin D and calcium suggests altered homeostasis, rather than true deficiency.