Differential Diagnosis for a 16-year-old Male with Low Vit D, High ALP, High PTH, and High LDH

Single Most Likely Diagnosis

• Vitamin D deficiency with secondary hyperparathyroidism: This condition is characterized by low vitamin D levels leading to increased parathyroid hormone (PTH) secretion, which in turn causes an increase in alkaline phosphatase (ALP) due to bone turnover. Elevated LDH can be seen in various conditions but in this context may be related to bone metabolism or other non-specific factors.

Other Likely Diagnoses

• Rickets or Osteomalacia: These conditions, caused by vitamin D deficiency, can lead to softening of the bones, elevated ALP due to bone healing process, and secondary hyperparathyroidism.
• Primary Hyperparathyroidism: Although less common in adolescents, it can cause elevated PTH, which leads to increased calcium levels, bone resorption, and consequently elevated ALP and potentially LDH.
• Paget's Disease of Bone: This condition is rare in adolescents but can cause localized bone destruction and remodeling, leading to elevated ALP and possibly LDH due to bone turnover.

Do Not Miss Diagnoses

• Malignancy (e.g., Lymphoma, Leukemia): Certain cancers can cause elevated LDH and may also affect vitamin D and calcium metabolism indirectly, leading to the observed lab abnormalities. It's crucial to rule out malignancy due to its severe implications.
• Sarcoidosis: This condition can cause elevated calcium levels due to vitamin D dysregulation, leading to secondary hyperparathyroidism, and may also affect liver function, potentially altering ALP and LDH levels.
• Hyperthyroidism: Although not directly linked to all the lab findings, hyperthyroidism can accelerate bone turnover, potentially leading to changes in ALP, PTH, and vitamin D levels.

Rare Diagnoses

• Familial Hypophosphatemic Rickets: A genetic disorder affecting phosphate reabsorption in the kidneys, leading to rickets or osteomalacia, and potentially causing the observed lab abnormalities.
• Vitamin D-dependent Rickets Type 1 or 2: Rare genetic disorders affecting the metabolism or action of vitamin D, leading to rickets or osteomalacia and associated biochemical abnormalities.
• Osteogenesis Imperfecta: A genetic disorder affecting collagen production, leading to bone fragility and potentially elevated markers of bone turnover like ALP.