What is the effect of increased ADH in SIADH?

In the context of Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH), increased levels of antidiuretic hormone (ADH) lead to significant physiological changes. One of the primary effects of elevated ADH is increased water reabsorption in the kidneys, particularly in the collecting ducts. This results in a higher retention of water in the body.

As more water is retained, the overall serum osmolarity decreases because there is an increase in the volume of water in relation to the amount of solute in the bloodstream. This dilution effect contributes to dilutional hyponatremia, where the sodium concentration in the blood becomes abnormally low due to the excess water. This phenomenon occurs despite the total amount of sodium in the body potentially being normal or even mildly increased, as the increased water alters the balance.

Thus, the correct choice effectively describes the pathophysiological outcome of SIADH, highlighting the relationship between increased ADH, water retention, and the resulting low sodium levels in the serum.