PTH and secondary hyperparathyroidism: Secondary hyperparathyroidism was also used as a definition of severe vitamin D deficiency in many studies, mostly based upon blood levels of PTH, with PTH values >7.6 pmol/L (64, 65, 67) or >5.5 pmol/L (59–61) as most commonly used, with PTH >6.9 pmol/L in one study (56).