What happens to the TSH (thyroid-stimulating hormone) blood level in hypothyroidism? Why is there enlargement of the thyroid in the endemic goiter disease?
When there is low T3 and T4 secretion by the thyroid the TSH secretion by the hypophysis is very stimulated and the TSH blood level increases. The increase in the TSH availability promotes the enlargement of the thyroid gland.
The thyroid enlargement is a reaction of a tissue that tries to compensate the functional deficiency by making the gland increase its size.