What is the primary effect of thyroxine?

Thyroxine, also known as T4, is a hormone produced by the thyroid gland that plays a crucial role in the regulation of metabolism within the body. The primary effect of thyroxine is to increase the rate of metabolism of fats, proteins, and carbohydrates. This hormone stimulates the metabolic processes, leading to an increase in the basal metabolic rate (BMR), which affects how quickly the body uses energy and processes nutrients.

When thyroxine levels are adequate, they enhance energy expenditure by promoting the breakdown of fats for energy (lipolysis), the synthesis of proteins (protein metabolism), and the metabolism of carbohydrates via the increase in glucose utilization. This hormone also influences various metabolic pathways, impacting how the body responds to different types of dietary energy sources.

In contrast, the other options relate to functions more closely associated with other hormones or systems in the body. For example, calcium regulation pertains primarily to parathyroid hormone and calcitonin, not thyroid hormones. Water retention in the kidneys is influenced by hormones like antidiuretic hormone (ADH) and aldosterone. The role of thyroid hormones in fat metabolism stands out as a significant, direct effect, solidifying option B as the correct response.