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Why can iodine deficiency cause an enlarged thyroid gland?

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USDA MyPlate dietary guidelines — human nutrition MyPlate, a revised set of dietary guidelines introduced by the U.S. Department of Agriculture (USDA) in 2011, divides the four basic food groups (fruits, grains, protein, and vegetables) into sections on a plate, the size of each section representing the relative dietary proportions of each food group. The small blue circle shown at the upper right illustrates the inclusion and recommended proportion of dairy products in the diet.

Iodine is an essential nutrient required for the production of thyroid hormones, specifically thyroxine and triiodothyronine. When dietary iodine intake is insufficient, the thyroid gland cannot produce adequate amounts of these hormones.12

The body regulates thyroid hormone levels through a negative feedback system involving the pituitary gland. When thyroid hormone levels in the blood drop, the pituitary gland compensates by increasing the secretion of thyrotropin (also known as thyroid-stimulating hormone, or TSH).3124

Thyrotropin serves two primary functions in this context: it stimulates the thyroid to capture any available iodine more efficiently to synthesize hormones, and it acts as a growth factor that stimulates the proliferation of thyroid cells. While this compensatory mechanism attempts to restore hormone production to normal levels, the chronic stimulation of thyroid cell growth leads to an enlargement of the gland, a condition known as goitre.124