Vitamin D deficiency: This is a common cause of low phosphorus levels, as vitamin D plays a crucial role in phosphorus absorption in the gut. A deficiency can lead to decreased phosphorus levels.

Primary hyperparathyroidism: Elevated parathyroid hormone (PTH) levels can cause increased excretion of phosphorus in the urine, leading to low serum phosphorus levels.
Renal tubular acidosis: This condition is characterized by impaired renal tubular function, which can result in excessive loss of phosphorus in the urine.
Malnutrition or starvation: Inadequate dietary intake of phosphorus can lead to low levels, especially in individuals with increased demand or malabsorption.
Alcoholism: Chronic alcohol consumption can lead to poor dietary intake, malabsorption, and increased renal excretion of phosphorus.

Diabetic ketoacidosis: Although not the most common cause, diabetic ketoacidosis can lead to low phosphorus levels due to increased renal excretion and shifts of phosphorus into cells. Missing this diagnosis can be life-threatening.
Sepsis: Sepsis can cause shifts of phosphorus into cells, leading to low serum levels. Early recognition and treatment of sepsis are critical.

Fanconi syndrome: A rare genetic disorder characterized by impaired renal tubular function, leading to excessive loss of phosphorus, glucose, and amino acids in the urine.
Oncogenic osteomalacia: A rare paraneoplastic syndrome associated with certain types of tumors, leading to low phosphorus levels due to increased renal excretion and impaired vitamin D activation.
X-linked hypophosphatemia: A rare genetic disorder characterized by impaired renal tubular reabsorption of phosphorus, leading to low serum phosphorus levels and rickets or osteomalacia.