Percentage of T4 and T3 released from thyroid gland-
**Core Concept**
The thyroid gland releases thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) into the bloodstream, playing a crucial role in regulating metabolism, growth, and development. The release of these hormones is tightly regulated by the hypothalamic-pituitary-thyroid axis.
**Why the Correct Answer is Right**
The majority of T4 (about 80%) is released from the thyroid gland, while a smaller percentage of T3 is released. This is because T4 is the primary hormone produced by the thyroid gland, and it is then converted to T3 in peripheral tissues by the enzyme 5'-deiodinase. The thyroid gland also releases a small amount of reverse T3 (rT3), which is an inactive metabolite of T3.
**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because it does not accurately reflect the relative proportions of T4 and T3 released from the thyroid gland.
* **Option B:** Incorrect because it implies that T3 is the primary hormone released by the thyroid gland, which is not true.
* **Option C:** Incorrect because it suggests that the percentage of T4 and T3 released is equal, which is not the case.
**Clinical Pearl / High-Yield Fact**
The thyroid gland releases a small amount of thyroxine-binding globulin (TBG) into the bloodstream, which binds to T4 and T3, regulating their transport and availability to peripheral tissues.
**Correct Answer: D. 80% of T4 and 20% of T3**