LabMed

Vitamin D Resistant Deficiency/Rickets

At a Glance

Vitamin D resistant rickets, typically due to renal phosphate wasting (X-linked hypophosphatemia), is the most common cause of rickets. In adults, it causes osteomalacia.

Vitamin D resistant rickets occurs almost exclusively in boys and resembles classic Vitamin D deficiency, therefore, it is usually recognized clinically. Vitamin D resistant rickets can usually be controlled by diet and active vitamin D (high phosphate with 1,25 dihydroxyvitamin D), so it is important to recognize and treat early before skeletal defects become severe.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The disorder is due to missing or inactive PHEX gene. PHEX inactivates phosphatonins that mediate phosphate wasting. Thus, patients with Vitamin D resistant rickets cannot retain phosphate, leading to renal phosphate wasting. Phosphate, calcium, and 1,25 hydroxyvitamin D levels should be measured. Patients with Vitamin D resistant rickets will have low serum phosphate, inappropriately normal 1,25 hydroxyvitamin D, and normal serum calcium and PTH levels.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Secondary causes of PHEX deficiency are not known. There are a large numbers of rare vitamin D-related disorders that might be considerations if there are atypical features to the case.

What Lab Results Are Absolutely Confirmatory?

PHEX sequencing is absolutely confirmatory, but it is rarely indicated.

Once diagnosis is clear, appropriate therapy with therapeutic response is the priority.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

Further testing includes follow-up serum chemistries (phosphate, calcium, PTH, 1,25 hydroxyvitamin D) to verify response to therapy and control of the disease.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Interference with medications is not common; however, the high phosphate diet without adequate 1,25 dihydroxyvitamin D might cause secondary hyperparathyroidism. PTH should be measured periodically in patients to rule out secondary hyperparathyroidism.

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