Parathyroid function

Magnesium ion is essential for normal Ca " and K" metaboHsm. In acute experimental magnesium deficiency in humans, hypocalcemia occurs despite adequate calcium intake and absorption and despite normal renal and parathyroid functions. Negative K" balance is also observed. AH biochemical and clinical abnormaHties disappear upon restoration of adequate amounts of magnesium to the diet (64). 

Some patients with reduced or absent parathyroid function, e.g., primary hypoparathyroidism, harbor anti-CaR antibodies (Fig. 2) [4]. In two patients, the antibodies were shown to activate the CaR as assessed by stimulation of MAPK and PLC. Tims, analogous to activating mutations of the CaR, anti-CaR antibodies can increase the sensitivity of the receptor to CaQ+, thereby resetting parathyroid and kidney to maintain hypocalcemia. 

Malabsorption Syndrome, with Special Reference to the Effects of Wheat Gluten (Frazer), 5, 69 Mellituria, Nonglucose (Sidbury), 4, 29 Microbiological Assay Methods for Vitamins (Baker and Sobotka), 5, 173 Organic Acids in Blood and Urine (Nordmann and Nordmann), 4, 53 Paper Electrophoresis Principles and Techniques (Peeters), 2, 1 Paper Electrophoresis of Proteins and Protein-Bound Substances in Clinical Investigations (Owen), I, 238 Parathyroid Function and Hyperparathyroidism, Biochemical Aspects of (Nordin), 4, 275... 

The parathyroid hormone content of blood has not been studied sufficiently to yield any data with regard to variation. The functioning of the glands is so closely related to other factors which regulate calcium and phosphorus metabolism that it is impossible to assign differences in these areas to variation in parathyroid function. The variation of the calcium (and phosphorus) in the blood has been noted (p. 55), and this variation, of course, may be due in a substantial degree to differences in parathyroid functioning. 

One of the reasons for suspecting that variations in parathyroid functioning are great is the fact that experimental animals have striking individual sensitivity to administered hormone. This fact has been noted particularly in dogs which have been used for assay animals. Also, in one study involving humans it was found that four normal individuals had their phosphate clearance increased 60 per cent, 190 per cent, 150 per cent, and 110 per cent, respectively, when 200 units of the hormone was administered intravenously, while two individu-... 

Ten patients who had taken lithium for less than 1 year and 13 who had taken it for more than 3 years were assessed for alterations in bone metabolism and parathyroid function (654). There were no differences in bone mineral density, serum calcium concentration, or PTH concentration, but both groups had increased bone turnover and the longterm group had nonsignificantly higher calcium and PTH concentrations (including one hyperparathyroid patient who had an adenoma excised). The authors conclusion that lithium therapy is not a risk factor for osteoporosis needs to be tempered by the small sample size, the case of adenoma, and the blood concentration trends. 

A lithium chloride solution caused changes in gravicur-vature, statocyte ultrastructure, and calcium balance in pea root, believed to be due to effects of lithium on the phosphoinositide second messenger system (678). The implications with regard to human parathyroid function are obscure. 

Kusalic M, Engelsmann F. Effect of lithium maintenance therapy on thyroid and parathyroid function. J Psychiatry Neurosci 1999 24(3) 227-33. 

In the case of osteoporosis that is not due to normal aging, but is secondary to another disease process, other laboratory examination may be necessary. Calcium blood level, thyroid, liver, and parathyroid function may need to be evaluated. Other diseases that cause secondary osteoporosis (such as gastrointestinal disease) are usually evident due to other symptomatology. 

In severe renal insufficiency there can be accumulation of magnesium due to reduced excretion, with inhibition of parathyroid function and consequent calcium and phosphorus abnormalities (12). 

Hypervitaminosis D apparently cannot arise from excessive exposure to sunlight but only occurs following inge.s-tion of large quantities of synthetic vitamin D for months The amount necessary has been estimated at 50,000 units or more in a person with normal parathyroid function. The mechanism may involve formation of excessive amounts of the vitamin D metabolite 25-OHD. Toxicity involves derangements of calcium metabolism, resulting in hypercalcemia and metastatic calcification of soft tissue. Most problems result from the hypercalcemia, which typically causes muscular weakness, anorexia, nausea, vomiting, and depression of the central nervous system (which can result in coma and death). In addition, deposition of calcium salts in the kidneys (nephrocalcinosis) and the tubules (nephrolithiasis)... 

Primary hyperparathyroidism is diagnosed by laboratory studies. Hypercalcemia should be documented by measuring total calcium and serum albumin, or ideally free calcium, on more than one occasion before initiating further testing. Measurement of intact PTH (with concomitant measurement of calcium) is the most sensitive and specific test for parathyroid function and is central to the differential diagnosis of hypercalcemia. Serum l,25(OH)2D is usually in the upper half of the reference interval or increased in primary hyperparathyroidism, as PTH stimulates its production. By contrast, l,25(OH)2D (lilte PTH) is. low-normal or suppressed in nonparathyroid hypercalcemia, except in sarcoidosis, other granulomatous diseases, and certain... 

Determination of PTH is useful in the differential diagnosis of both hypercalcemia and hypocalcemia for assessing parathyroid function in renal failure and for evaluating parathyroid function in bone and mineral disorders (see Calcium, Clinical Significance, Hypocalcemia, and Hypercalcemia Metabohc Bone Diseases and Interpretation of PTH Results). 

Figure 49-16 Intact PTH in assessing parathyroid function in end-stage renal disease. Dialysis patients were separated into those with early and advanced osteitis fibrosa, osteomalacia, and aplastic disease by quantitative histomorphometric analysis of undecalcified bone biopsies. (From Segre GV, Sherrard DJ, Pandian /WR, et al. Intact PTH (IRMA) II New applications to issues in parathyroid hormone and mineral metabolism. San Juan Capistrano, Calif Nichols Institute, 1989.)...

Eastell R, Yergey AL, Vierira NE, Cedel SL, Kumar R, Riggs BL. Interrelationship among vitamin D metabolism, true calcium absorption, parathyroid function, and age in women evidence of an age-related intestinal resistance to 1,25-dEiydroxyvitamin D action. J Bone Miner Res 1991 6 125-32. 

Gao P, Scheibel S, D Amour PD, John MR, Rao SD, Schmidt-Gayk H, et al. Development of a novel immunoradiometric assay exclusively for biologically active whole parathyroid hormone 1-84 implications for improvement of accurate assessment of parathyroid function. J Bone Miner Res 2001 16 605-14. 

Biochemical Aspects of Parathyroid Function and of Hyperparathyroidism B. E. C. Nordin... 

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