Water and Sodium Metabolism; Endocrine Genetics and Neoplasia Syndromes
Water and sodium
ADH concentrates urine via renal aquaporins. Central diabetes insipidus is ADH deficiency; nephrogenic diabetes insipidus is renal resistance. Both cause polyuria and dilute urine; central disease responds to desmopressin. SIADH causes euvolemic hyponatremia with high urine osmolality and low serum osmolality. Adrenal insufficiency causes hyponatremia with hyperkalemia. Correct sodium slowly to avoid osmotic demyelination, and avoid rapid correction of chronic hypernatremia.
Endocrine genetics and neoplasia
Recognize MEN1, with parathyroid, pituitary, and pancreatic tumors; MEN2A/2B, with RET mutations, medullary thyroid carcinoma, pheochromocytoma, and hyperparathyroidism; VHL; NF1; tuberous sclerosis; Li-Fraumeni; and Beckwith-Wiedemann. Screen at-risk relatives with targeted testing. Trap: medullary thyroid carcinoma requires calcitonin and RET testing; pheochromocytoma must be excluded before surgery.
Worked example
After craniopharyngioma resection, a child has polyuria, then hyponatremia, then polyuria again. This triphasic response reflects transient diabetes insipidus, SIADH from ADH release, and permanent diabetes insipidus. Monitor sodium and urine output closely and treat each phase.