Final answer:
T3 and T4 interact with target tissues similar to steroid hormones, crossing cell membranes and binding to intracellular receptors. They increase metabolism, carbohydrate absorption, energy use, and heat production. They also affect heart rate and enhance the sensitivity of cells to fight-or-flight hormones.
Step-by-step explanation:
T3 (tri-iodothyronine) and T4 (thyroxine) interact with target tissues in the same way as steroid hormones. They cross cell membranes and bind to intracellular receptors to regulate gene expression. However, unlike steroid hormones, thyroid hormones require special transporter proteins to cross cell membranes. Once inside the nucleus of cells, T3 and T4 turn on genes that control protein synthesis. They increase the rate of metabolism in cells, promote carbohydrate absorption, increase energy use, and generate more heat. Additionally, T3 and T4 increase the rate and force of the heartbeat and enhance the sensitivity of cells to fight-or-flight hormones.