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Why does thyroid gland enlarge both in hypothyroidism and hyperthyroidism?
Hypothyroid goitre is due to lack of iodine in diet and hyperthyroid goitre (Exopthalmic goitre, Grave's disease) is due to oversecretion of thyroxine, or Thyroid stimulating Hormone. How does these two apparently opposite effects produce similar physical manifestation of the disorder (symptom) i.e is the enlargement of the thyroid gland?

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I would bet it has to do with retaining water and/or minerals. –  fredsbend Jan 14 at 0:55
    
Check this out. It is a very similar question. –  biogirl Jan 15 at 13:37

1 Answer 1

The primary activity of the thyroid gland is to concentrate iodine from the blood to make thyroid hormone. The gland cannot make enough thyroid hormone if it does not have enough iodine. Therefore, with iodine deficiency the individual will become hypothyroid. Consequently, the pituitary gland in the brain senses the thyroid hormone level is too low and sends a signal to the thyroid. This signal is called thyroid stimulating hormone (TSH). As the name implies, this hormone stimulates the thyroid to produce thyroid hormone and to grow in size.

In Graves' disease one’s immune system produces a protein, called thyroid stimulating immunoglobulin (TSI). As with TSH, TSI stimulates the thyroid gland to enlarge producing a goiter. However, TSI also stimulates the thyroid to make too much thyroid hormone (causes hyperthyroidism). Since the pituitary senses too much thyroid hormone, it stops secreting TSH. In spite of this the thyroid gland continues to grow and make thyroid hormone. Therefore, Graves’ disease produces a goiter and hyperthyroidism.

Source: American Thyroid Association. What is a Goiter - Causes

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