Calorigenic hormone is
**Core Concept**
The calorigenic hormone is a substance that increases the body's metabolic rate, leading to an increase in heat production. This hormone plays a crucial role in thermogenesis, particularly in response to cold exposure or stress.
**Why the Correct Answer is Right**
The correct calorigenic hormone is Thyroxine (T4), a hormone produced by the thyroid gland. Thyroxine acts on the mitochondria to increase the rate of cellular respiration, resulting in an increase in ATP production and subsequently, an increase in heat production. This process is mediated through the activation of the thyroid hormone receptors, which ultimately leads to the increased expression of genes involved in thermogenesis.
**Why Each Wrong Option is Incorrect**
**Option A:** Epinephrine is a hormone involved in the 'fight or flight' response, but it primarily acts to increase glucose release from stored glycogen and increase cardiac output, rather than directly increasing metabolic rate.
**Option B:** Insulin is a hormone that lowers blood glucose levels by facilitating glucose uptake in cells, but it is not involved in increasing metabolic rate.
**Option C:** Parathyroid hormone (PTH) is involved in regulating calcium levels in the blood and bones, but it does not have a direct effect on metabolic rate.
**Clinical Pearl / High-Yield Fact**
Thyroid hormones are essential for regulating metabolic rate, and dysfunction can lead to conditions such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), which have significant effects on energy metabolism and overall health.
**Correct Answer: D. Thyroxine**