**Core Concept**
Thyroxine (T4) is a hormone that is synthesized in the thyroid gland. As a lipophilic hormone, it is not suitable for oral administration due to rapid degradation by the gut and liver. However, it is converted to its more active form, triiodothyronine (T3), by the enzyme 5'-deiodinase in peripheral tissues.

**Why the Correct Answer is Right**
Thyroxine is indeed orally active because it is a prodrug that requires conversion to its active form, T3, to exert its biological effects. This conversion occurs in the liver and other tissues, allowing the hormone to be effective even after oral administration. The enzyme 5'-deiodinase is responsible for this conversion, which is essential for the hormone's activity.

**Why Each Wrong Option is Incorrect**
**Option A:** TSH (Thyroid-Stimulating Hormone) is a hormone produced by the pituitary gland that stimulates the thyroid gland to produce thyroxine, but it is not orally active itself.
**Option C:** GH (Growth Hormone) is a hormone produced by the pituitary gland that stimulates growth and development, but it is not orally active and requires injection for administration.
**Option D:** Prolactin is a hormone produced by the pituitary gland that stimulates milk production in lactating women, but it is not orally active and also requires injection for administration.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that thyroxine is a prodrug that requires conversion to its active form, T3, to exert its biological effects. This is a classic example of a hormone that is not orally active in its native form but becomes effective after conversion in peripheral tissues.

**✓ Correct Answer: B. Thyroxine**