Question

Very short answer type.
How could an iodine-deficient diet lead to goitre?

Answer 1

Lack of iodine in the diet $\to$ diminished production of $T_3$ and $T_4$ $\to$ increased release of TSH $\to$ growth (enlargement) of thyroid gland $\to$ goitre.

Overexpression of TSH on thyroid leads to expansion of the gland. Immature thyroid hormones are produced excessively.