2025 – PAGE 221 – VITAMIN AND NUTRITIONAL DISORDERS
- NORMAL (or LOW) CALCIUM + LOW PHOSPHORUS
This pattern represents FAMILIAL HYPOPHOSPHATEMIC RICKETS (AKA “VITAMIN D RESISTANT RICKETS”). It is an X-linked DOMINANT renal disorder. The mutation causes excessive production of a growth factor that inhibits phosphate reabsorption in the proximal tubule. The treatment of choice is burosumab, a monoclonal antibody against this growth factor. Labs = Normal (or low) calcium, LOW serum phosphorus, HIGH alkaline phosphatase, normal Vitamin D 25, and NORMAL PTH since calcium is usually normal. Treat with oral phosphate supplementation and avoid hypOcalcemia by giving the active/oral form of Vitamin D (1,25).
- PEARL: For the exam, they will probably keep it simple and avoid giving you a low calcium level.
- NORMAL CALCIUM + LOW PHOSPHORUS
This represents INITIAL VITAMIN D DEPLETION. Low Vitamin D results in low phosphorus reabsorption. There is a compensatory increased PTH that temporarily normalizes calcium. Treat with Vitamin D supplementation, preferably Vitamin D3 (cholecalciferol).
PEARL: For the test, they probably want you to focus on FAMILIAL Hypophosphatemic Rickets. The differentiating lab would be a low Vitamin D level (25) in early Vitamin D depletion, versus a normal level in Familial Hypophosphatemic Rickets.
- LOW CALCIUM + LOW PHOSPHORUS
This represents SEVERE VITAMIN D DEFICIENCY resulting in poor absorption of calcium and phosphorus from the gut. PTH should be high. Treat with Vitamin D supplementation, preferably Vitamin D3 (cholecalciferol).
- LOW CALCIUM + HIGH PHOSPHORUS
This represents hypoparathyroidism, pseudohypoparathyroidism (high serum PTH but PTH resistance), or phosphorus overload.
- NORMAL CALCIUM + HIGH PHOSPHORUS
This represents renal disease, growth hormone excess, or a high phosphorus diet.
| LOW CALCIUM | NORMAL CALCIUM | |
| LOW PHOSPHORUS | Severe Vitamin D deficiency | Familial hypophosphatemic rickets (calcium can be low)
Early vitamin D deficiency |
| HIGH PHOSPHORUS | Hypoparathyroidism
Phosphorus overload Pseudohypoparathyroidism |
Renal disease
Growth hormone excess High phosphorus diet |
(DOUBLE TAKE) RICKETS OF PREMATURITY (AKA OSTEOPENIA OF PREMATURITY)
Premature babies have a high Vitamin D, calcium, and phosphorus requirement. Failure to administer these may result in Rickets of Prematurity. Treat with Vitamin D + Calcium + Phosphorus; you have to give all three, as Vitamin D alone is insufficient.
(DOUBLE TAKE) LIVER DYSFUNCTION
Liver dysfunction can result in decreased bile salts in the gut → Vitamin D absorption issues → Vitamin D deficiency.