PTH up-regulates 25-hydroxyvitamin D3 1-alpha-hydroxylase, the enzyme responsible for 1-alpha hydroxylation of 25-hydroxy vitamin D, converting vitamin D to its active form (1,25-dihydroxy vitamin D). This activated form of vitamin D increases the absorption of calcium (as Ca2+ ions) by the intestine via calbindin. PTH was one of the first hormones to be shown to use the G-protein adenylyl cyclase second messenger system. Regulation of serum phosphate
PTH reduces the reabsorption of phosphate from the proximal tubule of the kidney, which means more phosphate is excreted through the urine. However, PTH enhances the uptake of phosphate from the intestine and bones into the blood. In the bone, slightly more calcium than phosphate is released from the breakdown of bone. In the intestines, absorption of both calcium and phosphate is mediated by an increase in activated vitamin D. The absorption of phosphate is not as dependent on vitamin D as is that of calcium. The end result of PTH release is a small net drop in the serum concentration of phosphate. Vitamin D synthesis
PTH upregulates the activity of 1-α-hydroxylase enzyme, which converts 25-hydroxycholecalciferol, the major circulating form of inactive vitamin D, into 1,25-dihydroxycholecalciferol, the active form of vitamin D, in the kidney. Interactive pathway map
Regulation of PTH secretion
Secretion of parathyroid hormone is determined chiefly by serum ionized calcium concentration through negative feedback. Parathyroid cells express calcium-sensing receptors on the cell surface. PTH is secreted when [Ca2+] is decreased (calcitonin is secreted when serum calcium levels are elevated).
The level may be stated in pg/dL or pmol/L (sometimes abbreviated mmol/L); multiply by 0.1060 to convert from pg/dL to pmol/L.A US source states the average PTH level to be 8–51 pg/mL. In the UK the biological reference range is considered to be 1.6-6.9 pmol/L. Normal total plasma calcium level ranges from 8.5 to 10.2 mg/dL (2.12 mmol/L to 2.55 mmol/L). Interpretive guide
The intact PTH and calcium normal ranges are different for age; calcium is also different for sex. Model organisms
Model organisms have been used in the study of PTH function. A conditional knockout mouse line called Pthtm1a(EUCOMM)Wtsi was generated at the Wellcome Trust Sanger Institute. Male and female animals underwent a standardized phenotypic screen to determine the effects of deletion. Additional screens performed:  - In-depth immunological phenotyping

See also
Disorders of calcium metabolism
Parathyroid hormone family
Parathyroid hormone-related protein
Preotact

Footnote
References
Further reading
External links
 Media related to Parathyroid hormone at Wikimedia Commons
Parathyroid hormone: analyte monograph - the Association for Clinical Biochemistry and Laboratory Medicine
Overview of all the structural information available in the PDB for UniProt: P01270 (Parathyroid hormone) at the PDBe-KB.