Selective deficiency of 1,25-dihydroxycholecalciferol. A cause of isolated skeletal resistance to parathyroid hormone

Maternal Mineral and Bone Metabolism During Pregnancy, Lactation, and Post-Weaning Recovery

During pregnancy and lactation, female physiology adapts to meet the added nutritional demands of fetuses and neonates. An average full-term fetus contains ∼30 g calcium, 20 g phosphorus, and 0.8 g magnesium. About 80% of mineral is accreted during the third trimester; calcium transfers at 300-350 mg/day during the final 6 wk. The neonate requires 200 mg calcium daily from milk during the first 6 mo, and 120 mg calcium from milk during the second 6 mo (additional calcium comes from solid foods). Calcium transfers can be more than double and triple these values, respectively, in women who nurse twins and triplets. About 25% of dietary calcium is normally absorbed in healthy adults. Average maternal calcium intakes in American and Canadian women are insufficient to meet the fetal and neonatal calcium requirements if normal efficiency of intestinal calcium absorption is relied upon. However, several adaptations are invoked to meet the fetal and neonatal demands for mineral without requiring increased intakes by the mother. During pregnancy the efficiency of intestinal calcium absorption doubles, whereas during lactation the maternal skeleton is resorbed to provide calcium for milk. This review addresses our current knowledge regarding maternal adaptations in mineral and skeletal homeostasis that occur during pregnancy, lactation, and post-weaning recovery. Also considered are the impacts that these adaptations have on biochemical and hormonal parameters of mineral homeostasis, the consequences for long-term skeletal health, and the presentation and management of disorders of mineral and bone metabolism.

Pseudotumor cerebri in pseudohypoparathyroidism

A case of hypocalcemia with elevated serum PTH but normal renal response of cAMP and phosphate to PTH infusion is reported. The patient also had papilledema since many years probably due to long-lasting hypocalcemia.

Transient hypocalcemia with elevated serum parathormone in an infant of a hyperparathyroid mother

An infant presented at the age of 80 days with a hypocalcemic fit. The mother was found to have asymptomatic parathyroid adenoma. The infant's intact parathyroid hormone was high in the absence of hypomagnesemia, renal failure or overt rickets, which are known causes of secondary end-organ refractoriness. The patient was treated with calcium and 1-alpha-hydroxyvitamin D for only 2 weeks, but has remained asymptomatic and normocalcemic on follow-up to--the age of 2 years. It is believed that this has not been previously described and it is speculated that end-organ refractoriness, though brief, can account for all the symptoms.

Antiidiotypic PTH antibodies as a cause of elevated immunoreactive parathyroid hormone in idiopathic hypoparathyroidism, a second case: another manifestation of autoimmune endocrine disease?

A 69-year-old man became hypocalcemic under medical observation. The hypocalcemia occurred in the presence of circulating immunoreactive parathyroid hormone (PTH). Common causes of secondary hyperparathyroidism were excluded, as was PTH resistance using PTH infusions. The immunoreactive PTH was examined in detail. PTH immunoreactivity (1) was not retained on a C18 SPE-column, suggesting unusual molecular or physicochemical properties, unlike bona fide PTH; (2) was precipitated with 15% PEG, indicating a molecular size far in excess of native PTH; (3) had an apparent molecular size similar to immunoglobulins on size exclusion chromatography; (4) was retained on affinity chromatography with both Protein A and anti-hIgG antibodies. These data lead us to conclude that the immunoreactive PTH was due to antiidiotypic PTH autoantibodies. No significant quantities of true PTH were found in the patient's serum suggesting that his hypoparathyroidism was a result of PTH deficiency. Autoimmunity might explain the occurrence of both processes if an arrested antiidiotypic antibody cascade is assumed.

Familial hypoparathyroidism due to an abnormal parathyroid hormone molecule

The case of a 53-year-old man with familial hypoparathyroidism in the presence of circulating immunoreactive PTH is discussed. The patient responded to exogenous PTH by an increase in urinary cAMP excretion and by several post cyclase parameters including an increase in serum calcium and 1,25-dihydroxyvitamin D, an increase in urinary phosphate excretion and a decrease in urinary calcium. Immunoreactive PTH was detected in this patient's serum by three separate anti-PTH antisera. This immunoreactive PTH behaved aberrantly with these antisera. Nonparallelism to the standard curve was seen in two radioimmunoassays and the material was detected by an antiserum which preferentially binds bovine PTH. No circulating PTH binding activity was detectable. Family studies confirmed the genetic nature of the abnormality. HPLC studies revealed the presence of an abnormally hydrophobic fraction containing immunoreactive PTH. We believe these findings constitute strong evidence for the presence of an abnormal PTH molecule with reduced biological activity resulting in hypoparathyroidism.

A 6-hour human parathyroid hormone (1-34) infusion protocol: studies in normal and hypoparathyroid subjects

Parathyroid hormone (PTH)-resistant states are usually diagnosed by the failure of an acute PTH injection to elicit a rise in urinary cAMP and phosphate or, less commonly, by the failure of repeated PTH injections to raise serum calcium. We have established a 6 hour infusion of human PTH (1-34) which identifies PTH-resistant hypoparathyroid subjects on the basis of serum 1,25-dihydroxyvitamin D (1,25(OH)2D) and calcium responses. 1,25-Dihydroxyvitamin D levels increased by at least 58 pmol/liter and serum calcium by at least 0.1 mmol/liter in PTH-responsive hypoparathyroid subjects (n = 6), whereas in pseudohypoparathyroid subjects (n = 5) these levels rose by less than 22 pmol/liter and 0.06 mmol/liter respectively. The responsiveness of urinary phosphate excretion, expressed as the renal threshold phosphate concentration (TmPO4/GFR), to PTH also clearly separated the pseudohypoparathyroid patients from the other subjects. Differences in urinary calcium responses were observed though this parameter was less reliable in the identification of individual PTH-resistant or PTH-sensitive hypoparathyroid patients. Nephrogenous cAMP did not discriminate between groups when this protocol was used. This test has the potential to facilitate and extend the classification of PTH-resistant states.

Vitamin D2 therapy of pseudohypoparathyroidism. Assessment of non-compliance

A patient treated with massive doses of vitamin D for pseudohypoparathyroidism who had a convulsive episode due to hypocalcemia is reported. Evaluation of the patient's vitamin D status led us to conclude that the patient was non-compliant with vitamin D treatment.

[Hyperparathyroidism in hemodialysis patients: studies on parathyroid regulation and classification problems (author's transl)]

In a series of 22 patients undergoing chronic intermittent hemodialysis the response in secretion of parathyroid hormone (PTH) to an acute increase in serum calcium (Ca) concentration was studied. All patients had raised levels of predialysis PTH. Ca concentrations were measured by means of atomic absorption spectrophotometry and PTH concentrations were determined by radioimmunoassay. A close, significant, linear, negative correlation was demonstrated by regression analysis when Ca concentrations were correlated to changes in PTH, expressed as a percentage of the initial predialysis PTH value (100%). This finding supports the assumption that the function of the parathyroid gland at cellular level is similar in all patients, irrespective of very different predialysis PTH concentrations in peripheral blood. The results together with findings in literature were used to describe secondary hyperparathyroidism as a disease which shows typical phases during progression. Some still obscure phenomena of secondary hyperparathyroidism can be explained by discriminating between parathyroid function at a cellular and an organ level. The classification of hyperparathyroidism in regulative and autonomous forms only seems to be applicable when referred to the cellular level.

Pseudohypoparathyroidism and hypocalcemic "myopathy". A case report

A patient with the clinical features of pseudohypoparathyroidism and elevated concentrations of serum CK and LDH, which normalized after successful therapy, is described. Clinical signs of myopathy did not exist. The bioptical material from the m. tibialis anterior was microscopically normal. The biochemical analysis revealed a reduced phosphorylase-a-activity with the total phosphorylase-activity (a and b) being within the normal range. The significance of these findings as well as possible pathogenetic mechanisms are discussed.

Lack of response of 1,25-dihydroxycholecalciferol to exogenous parathyroid hormone in a patient with treated pseudohypoparathyroidism

The response of serum 1,25-dihydroxycholecalciferol (1,25-OH2CC) concentration to the administration of parathyroid extract (PTE) was studied in a patient with pseudohypoparathyroidism (PHP) type 1, 3 days after withdrawal of dihydrotachysterol (DHT) treatment. The patient had had a normal serum calcium (Ca), phosphorus (P) and immunoreactive parathyroid hormone (iPTH) level on DHT for 6 years. After PTE administration no rise of the 1,25-OH2CC concentration and no response of urinary cAMP and P were seen.

Demonstration of a lack of change in serum 1 alpha,25-dihydroxyvitamin D in response to parathyroid extract in pseudohypoparathyroidism

Studies were carried out to compare the effects of parathyroid extract (PTE) on serum and urinary calcium (Ca) and phosphorus (P), serum 25-hydroxyvitamin D (25-OHD), serum 24,25-dihydroxyvitamin D (24,25(OH)2D), serum 1 alpha,25-dihydroxyvitamin D (1 alpha,25(OH)2D), and urinary cyclic AMP in two normal subjects, two patients with hypoparathyroidism (HP) and six patients with pseudohypoparathyroidism (PHP), some of whom were on suboptimal treatment with vitamin D. Two of the patients with PHP were studied while on long-term treatment with 1 alpha,25-(OH)2D3. Before PTE, serum 1 alpha, 25(OH)2D was at the lower limit of normal in one patient and was abnormally low in the other five patients. None of these individuals was on treatment with 1 alpha,25(OH)2D3. Serum 25-OHD and 24,25(OH)2D were either increased or at the upper limit of normal in the patients given vitamin D and were normal in the other patients. PTE lowered the serum P and increased the serum 1 alpha,25(OH)2D, serum and urinary Ca, urinary P, and urinary cyclic AMP in the normal subjects and patients with HP. In individual studies, changes in serum 1 alpha,25(OH)2D and serum Ca occurred in parallel before, during, and after PTE. In contrast, PTE had very little effect in the patients with PHP. Whereas there were highly significant positive correlations between serum 1 alpha,25(OH)2D in each of the normal subjects and patients with HP, there were significant correlations in only one of the patients with PHP. An increase in serum Ca in response to PTE was observed in one of the two patients with PHP who were on long-term treatment with 1 alpha,25(OH)2D3. In these individuals, PTE produced only slight increases in serum 1 alpha,25(OH)2D. Serum 25-OHD and 24,25(OH)2D were not changed by PTE in any of the subjects or patients. The results provide evidence that hypocalcemia in HP and PHP arises in part from low circulating 1 alpha,25-(OH)2D, and indicate that the lack of change in serum 1 alpha,25(OH)2D with PTE in patients with PHP is related to impaired renal adenylate cyclase and phosphaturic responses. These and previous results support the idea that diminished renal production of 1 alpha,25(OH)2D, because of a defect in the parathyroid hormone-responsive adenylate cyclase system, may be a contributing factor in the pathogenesis of the abnormal calcium metabolism in PHP.

Hypoparathyroidism

Recent advances in our understanding of the physiologic actions of PTH and vitamin D have clarified certain aspects of the pathogenesis, classification, and management of hypoparathyroidism. Central to pathogenesis and categorization is the recognition that hypoparathyroidism may result from PTH deficiency, ineffectiveness, or resistance, with a resultant inability to stimulate adenylate cyclase in target tissues. This aberration in adenylate cyclase activity impairs certain physiologic responses such as renal phosphate excretion and renal calcium reabsorption that are required for proper calcium homeostasis. Also critical is the subnormal production of 1 alpha,25-dihydroxycholecalciferol (1,25-DHCC). Although the precise mechanism for the deficiency of 1,25-DHCC remains unclear, one may hypothesize that in hormone-deficient or hormone-ineffective hypoparathyroidism, decreased synthesis results from the absence of the two recognized stimuli for 1 alpha-hydroxylase--bioactive PTH and hypophosphatemia. Provision of either one of these stimuli would then be expected to restore 1,25-DHCC to normal levels, which could explain the calcemic response to PTH in these patients. There is some evidence that the synthesis of 1,25-DHCC may be "primarily" affected in PTH-resistant hypoparathyroidism, and thus may be unresponsive to any of the known stimuli. It remains conceivable, however, that during normocalcemic phases, such patients may improve their renal cyclic AMP and phosphaturic responses to PTH, with associated improvement in 1,25-DHCC synthesis. Certain acquired forms of PTH resistance such as hypomagnesemia and end-stage renal disease may also be associated with defective 1-hydroxylation. Whether occurring primarily or as a secondary process, the subnormal production of 1,25-DHCC may influence calcium and skeletal metabolism directly or by modifying response to PTH. The availability of 1,25-DHCC provides an effective and physiologically meaningful mode of therapy for most cases of hypoparathyroidism.