UWorld Endocrine Practice Test 2025 - Free Endocrine Practice Questions and Study Guide

In patients with the syndrome of inappropriate antidiuretic hormone (SIADH), the body produces excess antidiuretic hormone (ADH) which leads to an inappropriate retention of water. When this occurs, it dilutes the serum, resulting in a decrease in serum osmolality. Consequently, the serum becomes hypoosmolar.

In SIADH, as more water is retained in the body, the urine becomes more concentrated due to the effects of retained water. This water retention leads to a low serum sodium concentration, which also contributes to the decreased osmolality in the serum. Therefore, a decrease in serum osmolality is expected in patients suffering from SIADH due to the excess water in the circulation.

Other lab results would typically align with this, such as low urine osmolality and high urine specific gravity because the urine is concentrated as a result of the high levels of ADH. Understanding this relationship between ADH and serum osmolality is crucial in managing and diagnosing conditions associated with fluid balance.