Does amino acid sensing by the CaSR primarily affect PTH secretion under conditions of protein deficiency and reductions in plasma amino acid levels as suggested by the phenomenon of secondary hyperparathyroidism in subjects on low protein diets (reviews: Conigrave et al., 2002, 2008) or does it act primarily to suppress PTH secretion under conditions of protein excess as suggested by parathyroid cell responses in vitro (Conigrave et al., 2004). Here, PTH is linked to protein S deficiency.