PTH and vitamin D deficiency: After adjusting for serum 25(OH)D and GFR, age over 40 years was significantly associated with a 5.5 pg/mL increase in serum PTH in adult subjects with normal renal function (GFR≥60 mL/min/1.73m2) and no vitamin D deficiency (25(OH)D≥12 ng/mL).