Which of the following forms of thyroid hormone is most readily found in the circulation?
**Core Concept**
Thyroid hormones are produced in the thyroid gland through the iodination of thyroglobulin, a process involving the enzyme thyroid peroxidase (TPO). The primary forms of thyroid hormone in the circulation are thyroxine (T4) and triiodothyronine (T3), with T4 being the most abundant.
**Why the Correct Answer is Right**
Thyroxine (T4) is the predominant form of thyroid hormone in the circulation because it is produced in greater quantities than triiodothyronine (T3). T4 is synthesized in the thyroid gland through the iodination of tyrosine residues on thyroglobulin, resulting in the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT). Two DIT molecules are then coupled to form T4. T4 is then released into the circulation, where it is converted to T3 by the enzyme 5'-deiodinase.
**Why Each Wrong Option is Incorrect**
* **Option B:** Reverse T3 (rT3) is a metabolically inactive form of T3, produced through the deiodination of T4. While rT3 is present in the circulation, it is not the most abundant form of thyroid hormone.
* **Option C:** Free T4 (FT4) is the biologically active form of T4, which is not bound to proteins in the circulation. While FT4 is an important measure of thyroid function, it is not the most readily found form of thyroid hormone in the circulation.
* **Option D:** This option is blank and cannot be evaluated.
**Clinical Pearl / High-Yield Fact**
Thyroid hormones play a crucial role in regulating metabolism, growth, and development. The conversion of T4 to T3 is an important step in the activation of thyroid hormone, and any disruption in this process can lead to hypothyroidism or hyperthyroidism.
**Correct Answer: C. T4**