Which hormone is primarily responsible for fluid balance in the body?

Aldosterone plays a crucial role in regulating fluid balance within the body. It is a steroid hormone produced by the adrenal glands, specifically in the zona glomerulosa. Aldosterone is essential for maintaining sodium and potassium levels, as it promotes sodium reabsorption and potassium excretion in the kidneys.

By facilitating the reabsorption of sodium, aldosterone indirectly influences water retention, because water follows sodium osmotically. This process helps maintain blood volume and blood pressure, thereby ensuring that the body's fluid balance is preserved. An increase in aldosterone leads to more sodium being retained, which in turn leads to increased water retention, assisting the body in managing its overall fluid balance.

In contrast, insulin is primarily involved in glucose metabolism and regulating blood sugar levels, while thyroxine influences overall metabolism and growth; neither of these hormones directly impacts fluid balance. Glucagon primarily works to raise blood glucose levels during fasting states and also does not directly regulate fluid balance. Therefore, the primary responsibility for fluid balance in the body lies with aldosterone.