Why is thyroglobulin described as a prohormone?

Thyroglobulin is described as a prohormone because it serves as the essential precursor and structural scaffold for the synthesis of thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3).123

Within the thyroid gland, thyroglobulin is stored in the follicles as a primary component of colloid. The synthesis process involves the attachment of iodine to tyrosine residues located within the thyroglobulin molecule. When the body requires thyroid hormones, thyroglobulin is reabsorbed from the colloid into the follicular cells, where it is enzymatically hydrolyzed (split) to release the active T4 and T3 hormones into the circulation. Thus, thyroglobulin acts as both a storage reservoir and a necessary intermediate that must be processed to produce biologically active hormones.134