Why do goiters occur?

I am sorry to hear that your son has a bump on the back of his neck. I would definitely recommend that he have the lump on the back of his neck examined by a physician (primary care physician, or ENT aka ears nose throat physician). However with that being said, I am happy to give you some general information about goiter. A goiter is a general term that refers to significant thyroid swelling/growth. The thyroid gland is an endocrine gland that sits over the trachea (wind pipe) beneath the larynx (voice box) in the anterior neck. It normally secretes thyroid hormone (T3 and T4) which acts to upregulate or downregulate the bodies metabolism (the "gas pedal" of metabolism). Thyroid hormone is synthesized with iodine as one of the main components, which explains why the dysfunction resulting in (sometimes severe) thyroid swelling/overgrowth is most commonly seen in the setting of iodine deficiency. This is also why underdeveloped (aka "third-world") countries have a higher incidence of iodine deficiency goiter. Iodine is put in many different foods, including iodized table salt, which is why iodine deficient thyroid goiter is generally not very common in the United States. I don't have a great explanation as to what might be happening in the posterior aspect of your son's neck, but it doesn't sound like a goiter. I would definitely recommend having a physician take a look at it. Best of luck. I hope this helps.