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Autoimmune polyglandular syndrome type 1 (NORD)
Adrenal insufficiency: Pathology review
Adrenal masses: Pathology review
Cushing syndrome and Cushing disease: Pathology review
Diabetes insipidus and SIADH: Pathology review
Diabetes mellitus: Pathology review
Hyperthyroidism: Pathology review
Hypopituitarism: Pathology review
Hypothyroidism: Pathology review
Multiple endocrine neoplasia: Pathology review
Neuroendocrine tumors of the gastrointestinal system: Pathology review
Parathyroid disorders and calcium imbalance: Pathology review
Pituitary tumors: Pathology review
Thyroid nodules and thyroid cancer: Pathology review
Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
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|Serum osmolality||240 mOsm/kg|
|Urine osmolality||445 mOsm/kg|
How does your body know when to retain fluids and when to get rid of them?
It’s not like you just think to yourself “I’ve had too much water, better get rid of some.” (If you do, and it works, call us).
Anyways, the more ADH floating around in your blood, the more fluid you retain.
The less ADH in your blood, the more fluid you excrete.
These tubes though also allow fluids and electrolytes to move through the tube walls and back into the blood if needed.
ADH affects the last two-thirds of these tubes, called the distal convoluted tubule and the collecting ducts.
These tubes focus almost exclusively on reabsorbing water back into the blood.
The wall of these tubes are unsurprisingly made up of cells, a common trait of living things, but these cells have proteins called aquaporins.
Aquaporins allow water to move quickly in and out of the cells.
The more ADH floating around in the blood, the more aquaporins are available to... ahem...facilitate water movement through the cell (yo, wata, come over here for a sec).
So when ADH is low, most of the water flows through the distal convoluted tubule and the collecting duct, giving us diluted urine.
When ADH is high, aquaporins grab much of the water passing through the these tubes and throws them back into the blood.
When I drink a glass of water and that water is absorbed into my blood, my plasma osmolality drops, which means I’m diluting my blood with the water.
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterized by excessive release of antidiuretic hormone from the posterior pituitary gland or another source. The increase in fluid retention often results in dilutional hyponatremia in which the plasma sodium levels are lowered. SIADH may present with euvolemic hyponatremia with continued urinary sodium excretion. Urine osmolality is usually higher than serum osmolality. Very low serum sodium levels can lead to cerebral edema or seizures. Other symptoms of SIADH include fatigue, confusion, muscle weakness, nausea, vomiting, lack of appetite, and weight loss. Treatment usually involves fluid restriction, salt tablets, IV hypertonic saline, diuretics, and drugs like conivaptan, tolvaptan, or demeclocycline.
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