Dietary iodine deficiency enlarges the thyroid gland. What accumulates inside it?

1. Iodine is the essential component of triiodothyronine and thyroxine, so a deficiency stops their synthesis. 2. Low thyroid hormone removes the negative feedback signal, so thyroid-stimulating hormone is secreted in rising amounts. 3. That hyperstimulation makes thyroglobulin pile up in the follicles as extra colloid. 4. The accumulated colloid swells the whole gland, and the visible swelling is a goiter. 5. Calcitonin comes from parafollicular cells but is not stored as bulk in them. 6. Neither pooled blood nor fat deposits play any part in this mechanism. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 17 "The Endocrine System", section 17.4 The Thyroid Gland_