Methods are described for monitoring the amounts of PTH variants in a biological sample by digesting the sample to produce surrogate peptides specific to the targeted PTH variants, and detecting and quantifying the surrogate peptides by selective reaction monitoring (SRM) mass spectrometry, using a set of precursor-to-product ion transitions optimized for sensitivity and selectivity. The PTH variants, or a portion thereof, may be concentrated in the sample by means of immunoaffinity capture or other suitable technique. The mass spectrometric method described herein enables the concurrent measurement of peptides representative of a plurality of targeted PTH variants in a single assay.