Thyroid peroxidase is required for all of the following steps in thyroid hormones synthesis except:
**Core Concept**
Thyroid peroxidase is a crucial enzyme in the synthesis of thyroid hormones, specifically involved in the oxidation of iodide and the coupling of iodotyrosine residues to form triiodothyronine (T3) and thyroxine (T4).
**Why the Correct Answer is Right**
Thyroid peroxidase (TPO) catalyzes the iodination of tyrosine residues on thyroglobulin, the protein scaffold for thyroid hormone synthesis. This enzyme is essential for the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT) from tyrosine. TPO also facilitates the coupling of two DIT molecules to form T4, and one DIT and one MIT to form T3. The correct answer will be the step that does not require TPO.
**Why Each Wrong Option is Incorrect**
**Option A:** Iodide uptake into the thyroid gland is the first step in thyroid hormone synthesis, but it does not require TPO. Iodide is transported into the thyroid gland via the sodium-iodide symporter (NIS), where it is then oxidized by TPO.
**Option B:** The coupling of two DIT molecules to form T4 requires TPO, as it involves the oxidation of the tyrosine residues and the formation of the iodothyronine bond.
**Option C:** The formation of MIT from tyrosine also requires TPO, as it involves the iodination of the tyrosine residue.
**Clinical Pearl / High-Yield Fact**
Thyroid peroxidase antibodies are a hallmark of autoimmune thyroiditis, such as Hashimoto's thyroiditis, and can lead to hypothyroidism if left untreated.
**Correct Answer: A. Iodide uptake into the thyroid gland.