Co-existence of Congenital Adrenal Hyperplasia and Familial Hypokalemic Periodic Paralysis due to and Pathogenic Variants.

Steroid 21-hydroxylase deficiency is the most common cause of congenital adrenal hyperplasia (CAH), usually due to biallelic variants in . Classical 21-hydroxylase deficiency is characterised by virilisation of the external genitalia in females and hypocortisolism. Hyponatremia and hyperkalemia are among the common biochemical findings. Familial hypokalemic periodic paralysis (FHPP) is a rare disorder in which affected individuals may experience paralytic episodes associated with hypokalemia, caused by pathogenic variants in or . A 14-year-old female, who had been diagnosed with classical 21-hydroxylase deficiency and treated with hydrocortisone and fludrocortisone since early infancy, presented with acute onset weakness. The laboratory results revealed a remarkably low serum potassium level. The family history revealed that both her father and uncle had the same hypokalemic symptoms, which suggested an FHPP diagnosis. We found two previously reported homozygous variants in the (p.Ile173Asn) and (p.Arg672His) genes in the patient. Therefore, diagnoses of simple virilising 21-hydroxylase deficiency and FHPP were genetically confirmed. Here, FPHH and chronic overtreatment with fludrocortisone may explain the presentation of our patient with severe hypokalemia. The family’s medical history, which is always a valuable clue, should be investigated in detail since rare inherited conditions may co-occur in geographies where consanguineous marriages are common and the genetic pool is diverse. In patients with CAH, care should be taken to avoid overtreatment with fludrocortisone. Androgens may have triggered the hypokalemic attack in FHPP, as supported in a previous study.