Symptomatic heterozygous Gitelman syndrome in a professional baseball player: a case report.

Gitelman syndrome (GS) is an inherited renal salt wasting tubulopathy caused by variants in SLC12A3 and classically associated with hypokalemic metabolic alkalosis, hypomagnesemia, and hypocalciuria. We describe an 18-year-old professional baseball player with an approximately 6-month history of recurrent, exertional muscle cramping, fatigue, and profuse sweating that persisted despite hydration and commercial electrolyte supplementation. Laboratory evaluation demonstrated blood urea nitrogen (BUN) levels of 24-27 mg/dL, creatinine levels of 1.20-1.38 mg/dL, bicarbonate of 29-31 mmol/L, and 24-h urinary sodium of 163 mmol/L. The serum potassium measurements did not indicate levels below 3.5 mmol/L. Persistent symptoms and the urinary sodium finding raised concern for abnormal renal salt handling. Evaluation was complicated by limited, episodic access to the athlete while he trained and competed in the Dominican Republic, restricting opportunities for serial biochemical testing. Genetic sequencing identified a heterozygous SLC12A3 gene c.2990C>T variant. Management included individualized potassium and magnesium supplementation, naproxen sodium 220 mg daily, and pre-exercise iontophoresis to reduce sweating. At the 6-month follow-up, the athlete reported less cramping and improved endurance; BUN was 17.4 mg/dL, creatinine was 1.20 mg/dL, and potassium was 4.59. This case highlights the diagnostic challenge posed by an incomplete GS phenotype in a high-performance athlete and the importance of correlating clinical, biochemical, and genetic findings. Heterozygous SLC12A3-associated salt-wasting physiology should be considered in athletes with persistent compatible symptoms and evidence of inappropriate renal electrolyte loss, while alternative acquired and endocrine causes are evaluated.