Evidence of a parathyroid hormone-independent chronic effect of estrogen on renal phosphate handling and sodium-dependent phosphate cotransporter type IIa expression

Recurrent persistent hypophosphatemia in a male-to-female transgender patient with anorexia nervosa: Case report

Refeeding hypophosphatemia remains a serious and common complication during the early phases of nutritional rehabilitation and weight restoration for patients with anorexia nervosa. Typically, the risk of refeeding hypophosphatemia diminishes after the first 1-2 weeks of the refeeding process and thus, frequent monitoring serum phosphorus levels becomes less important as refeeding proceeds. Herein, we describe a case of persistent recurrent hypophosphatemia in a male-to-female transgender patient with severe anorexia nervosa. As transgender patients become increasingly seen in the realm of eating disorders, it is worth noting this phenomenon to mitigate adverse events.

Regulation of serum phosphate

The regulation of serum phosphate, an acknowledged risk factor for chronic kidney disease and cardiovascular mortality, is poorly understood. The discovery of fibroblast growth factor 23 (FGF23) as a key regulator of renal phosphate handling and activation of vitamin D has revolutionized our comprehension of phosphate homeostasis. Through as yet undetermined mechanisms, circulating and dietary phosphate appear to have a direct effect on FGF23 release by bone cells that, in turn, causes renal phosphate excretion and decreases intestinal phosphate absorption through a decrease in vitamin D production. Thus, the two major phosphaturic hormones, PTH and FGF23, have opposing effects on vitamin D production, placing vitamin D at the nexus of phosphate homeostasis. While our understanding of phosphate homeostasis has advanced, the factors determining regulation of serum phosphate level remain enigmatic. Diet, time of day, season, gender, age and genetics have all been identified as significant contributors to serum phosphate level. The effects of these factors on serum phosphate have major implications for what is understood as 'normal' and for studies of phosphate homeostasis and metabolism. Moreover, other hormonal mediators such as dopamine, insulin-like growth factor, and angiotensin II also affect renal handling of phosphate. How the major hormone effects on phosphate handling are regulated and how the effect of these other factors are integrated to yield the measurable serum phosphate are only now beginning to be studied.

Autosomal dominant hypophosphatemic rickets in an 85 year old woman: characterization of her disease from infancy through adulthood

BACKGROUND

Autosomal dominant hypophosphatemic rickets (ADHR) is a rare genetic disorder of phosphate homeostasis characterized, when severely expressed, by osteomalacia, suppressed levels of calcitriol, and renal phosphate wasting due to elevated levels of fibroblast growth factor 23 (FGF23). The disease is caused by heterozygous FGF23 mutations at the RXXR site that prevent cleavage of the intact hormone.

OBJECTIVES

An FGF23 mutation was identified in the proband an 85-year-old woman with elevated FGF23 levels, and her clinical course was characterized. Medical records revealed she was treated for rickets as an infant. She was then asymptomatic until soon after her 4th pregnancy, when she suffered incapacitating bone pain and weakness, age 37. Symptoms remitted with brief treatment.

RESULTS

The proband and one son, but not other family members, were found to be heterozygous for the R176Q mutation in FGF23. Expression of this germ line mutation was strikingly different in both individuals in terms of skeletal health, FGF23 levels and disease activity.

CONCLUSIONS

The identified FGF23 mutation in two members of this family raises questions about molecular mechanisms that have led to intermittent increases in FGF23 synthesis and secretion, and disease expression.