Vitamin D-dependent Rickets Type 1A Mimicking Pseudohypoparathyroidism in Presence of Active Tuberculosis

Vitamin D-dependent rickets type 1A is caused by pathogenic variants of CYP27B1 gene, which is inherited in autosomal recessive pattern. These variants lead to defective 1α-hydroxylase enzymatic activity, leading to impaired renal formation of 1,25(OH)₂ vitamin D. We report a case of a 16-year-old Asian male patient, with short stature and progressive bone deformity, whose biochemical parameters revealed low levels of 1,25(OH)₂ vitamin D, low serum calcium levels, along with high phosphorus and raised levels of intact parathyroid hormone. These biochemical parameters suggested the diagnosis of pseudohypoparathyroidism. The patient also had concurrent extrapulmonary tuberculosis during the time of presentation to our endocrine unit. However, on molecular testing, it was revealed that the patient was harboring pathogenic variants of the CYP27B1 gene, in a compound heterozygous manner, with a novel missense mutation in exon 6 of the CYP27B1 gene, c.1136G > C (p.Arg379Thr), suggesting the diagnosis of vitamin D-dependent rickets type 1A. The cause of high phosphorus at the time of presentation, which led to a diagnostic dilemma of pseudohypoparathyroidism, was later explained by presence of active extra pulmonary tuberculosis. This report describes a case of vitamin D-dependent rickets type 1A, mimicking pseudohypoparathyroidism owing to presence of concurrent illness like extrapulmonary tuberculosis.