Before we state what causes a goiter, we will briefly describe what it is. Basically a goiter is too much tissue growth of the thyroid. It is not a swelling of the thyroid like many think. In the US the most common cause of this is people with Hashimoto's going undiagnosed for so long. It is not from iodine deficiencies. Natural health care practitioners typically go this route first or blame the issue on the consumption of too many goitrogenic foods. This is just not the case, they are not testing their patients, or they do not understand how to read the test results properly and are putting people under a blanket diagnosis to make it easier to treat.
The goiter can and will form when there is too much TSH or even HCG present in the body. If your thyroid gland is exposed to too much of either of these, you will likely end up with a goiter.
Iodine deficiency can cause a goiter to form, but here in the US that's very rare. So make sure you have a health care practitioner that knows how to read thyroid test panels properly and understands exactly what causes goiters. If you're not being properly diagnosed, you will not be properly treated. If you're not properly treated, your condition will likely worsen.