What are the thyroid hormones T3 and T4 primarily responsible for?

The thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) play a crucial role in regulating the body's metabolic activity. They influence how the body uses energy and how it responds to other hormones and biological processes, effectively serving as key regulators of metabolism. This includes the modulation of the metabolic rate, which impacts how quickly or slowly the body burns calories and utilizes nutrients for energy. T3 is the more active form of the hormone, and both hormones are vital for ensuring proper growth, development, and overall energy balance within the body.

In contrast, the other options do not accurately reflect the primary functions of T3 and T4. Regulating digestive enzymes relates more to digestive hormones and not directly to thyroid hormones. Enhancing muscle growth is influenced by a variety of factors, including anabolic hormones like testosterone and growth hormone, rather than primarily by thyroid hormones. Maintaining fluid balance is controlled by hormones such as aldosterone and antidiuretic hormone (ADH), rather than being a direct function of thyroid hormones.