PTH and primary hyperparathyroidism: Furthermore, low 25(OH)D may occur from increased 25(OH)D to 1,25(OH)2D conversion by increased PTH in primary hyperparathyroidism and during treatment with PTH (teriparatide) (38-42); this can occur due to increased activity of renal 1-alpha-hydroxylase (43), the enzyme responsible for this conversion.