The thyroid hormone thyroxine (T4) is derived from?
**Core Concept**
Thyroxine (T4) is a thyroid hormone synthesized and secreted by the thyroid gland, playing a crucial role in regulating metabolism, growth, and development. The production of T4 involves a series of enzymatic reactions and iodination of tyrosine residues in thyroglobulin.
**Why the Correct Answer is Right**
Thyroxine is derived from tyrosine residues in thyroglobulin, a large protein molecule produced by the thyroid gland. The process of T4 synthesis involves the iodination of tyrosine residues, which are then coupled to form T4. This process is catalyzed by thyroid peroxidase (TPO), an enzyme present in the thyroid gland. The iodination of tyrosine is a critical step in the synthesis of T4, as it allows for the formation of the iodothyronine ring structure.
**Why Each Wrong Option is Incorrect**
**Option A:** While thyroglobulin is the protein molecule from which T4 is derived, it is not the correct answer as it is the substrate, not the precursor.
**Option B:** Iodine is essential for the synthesis of T4, but it is not the direct precursor of T4.
**Option C:** Triiodothyronine (T3) is a thyroid hormone, but it is not the precursor of T4; rather, it is synthesized from T4 through deiodination.
**Clinical Pearl / High-Yield Fact**
It is essential to note that the synthesis of T4 involves the iodination of tyrosine residues in thyroglobulin, a process that is dependent on the presence of iodine and thyroid peroxidase.
**Correct Answer:** D. Tyrosine