Serum phosphorus levels in isolation are of limited diagnostic value and should be correlated with serum calcium levels. An increased phosphorus with decreased calcium suggests either hypoparathyroidism or renal disease. A decreased phosphorus and an increased calcium suggests hyperparathyroidism or sarcoidosis. When both calcium and phosphorus are decreased, diagnostic considerations include malabsorption, vitamin D deficiency and renal tubular acidosis. Increased phosphorus and normal or increased calcium suggests milk-alkali syndrome or hypervitaminosis D.