Which hormone is critical for the regulation of metabolism and body temperature?

Thyroid hormones, particularly thyroxine (T4) and triiodothyronine (T3), play a critical role in regulating metabolism and maintaining body temperature. These hormones are produced by the thyroid gland and influence almost every cell in the body by modulating the metabolic rate. Higher levels of thyroid hormones increase the basal metabolic rate, which leads to enhanced energy expenditure and heat production. This thermogenic effect is vital for thermoregulation, especially in response to cold environments, as an increase in metabolism contributes to heat generation.

Thyroid hormones also regulate the metabolism of carbohydrates, fats, and proteins, which further emphasizes their significance in overall metabolic processes. Insufficient production of these hormones, a condition known as hypothyroidism, can lead to reduced metabolic activity and difficulty maintaining body temperature, often resulting in symptoms such as fatigue, weight gain, and feeling cold. Conversely, excess thyroid hormone production, or hyperthyroidism, can elevate metabolism and lead to weight loss, increased body temperature, and other systemic effects.

In summary, thyroid hormones are essential for metabolic regulation and thermoregulation, making them the correct answer to the question.