**Core Concept**
Thyroid hormone storage occurs in a precursor protein form synthesized and secreted by thyroid follicular cells. The primary storage form is not the active hormone itself but a large glycoprotein that contains iodinated tyrosine residues, which are later cleaved to release active thyroid hormones.

**Why the Correct Answer is Right**
Thyroglobulin is the major storage protein in the thyroid follicle. It is synthesized and secreted into the follicular lumen, where it binds iodine and tyrosine residues to form thyroglobulin, which contains both T3 and T4 precursors. Active thyroid hormones (T3 and T4) are released through proteolytic cleavage of thyroglobulin by thyroid peroxidase. This process ensures a reservoir of hormones that can be rapidly mobilized when needed.

**Why Each Wrong Option is Incorrect**
Option A: Tri-iodo tyrosine (T3) is an active thyroid hormone, not a storage form. It is released from thyroglobulin, not stored as such.
Option B: Tri-iodothyronine (T3) is the biologically active form of thyroid hormone and is not stored in large quantities; it is synthesized and released directly.
Option D: Di-iodo tyrosine (DIT) is a precursor molecule used in hormone synthesis but is not stored; it is incorporated into thyroglobulin and later processed.

**Clinical Pearl / High-Yield Fact**
Thyroglobulin is a sensitive biomarker in thyroid cancer monitoring. Elevated serum thyroglobulin levels after thyroidectomy indicate residual or recurrent disease, especially in differentiated thyroid cancer.

✓ Correct Answer: C. Thyroglobulin