Goiter

A goiter is an enlarged thyroid. The thyroid is the gland that makes thyroid hormone, and when you have a goiter, it takes up more room than normal. Sometimes you can see it, sometimes it can be felt on palpation, which means examining it with the fingers, and other times it shows up on an imaging study, a test that takes pictures of the inside of the body.

The interesting part is why it grows. When something gets in the way of making thyroid hormone, hormone levels can drop. If they do, the pituitary gland, a gland that sends signals to the thyroid, responds by putting out more TSH, the hormone that stimulates the thyroid. That signal reaches the follicular cells, the thyroid cells it acts on, and makes them multiply. This is called hyperplasia. More cells make a bigger gland, and that growth is the goiter: the thyroid's attempt to make up for the shortfall. Picture a shop that can't keep up with its orders. The manager hires more workers, and the shop has to expand to make room for all of them.

Around the world, the most common thing that holds back hormone production is a lack of iodine. And here's something that surprises many people: the vast majority of people with a goiter have normal thyroid function. Even though the gland is bigger, the hormone it makes stays at normal levels.