The thyroid gland needs iodine to produce the thyroid hormones thyroxine and triiodothyronine. If iodine is lacking in the diet, the thyroid initially cannot produce enough thyroid hormones. Low levels of thyroid hormones cause the thyroid gland to grow as it works harder to absorb as much available iodine from the bloodstream as possible in order to make more thyroid hormones.
A form of hyperthyroidism is called toxic nodular goiter or toxic thyroid adenoma. Adenomas, abnormal nodules of tissue in the thyroid, constantly produce thyroid hormones even when they are not needed. ”Toxic” means that the thyroid is producing thyroid hormone at an increased rate. If the increased hormone production is coming from a single nodule in the gland, this is called toxic adenoma (which appears clinically as a lump). If there are many nodules causing the hyperthyroidism, this is referred to as multinodular goitre.
The causes of toxic nodular goiter are unknown. The nodules typically grow from a simple goiter, an enlarged thyroid that occurs when the thyroid gland reacts to low hormone levels by growing to increase hormone production. In some cases, iodine injected into the blood during a CT scan or heart catheterization may trigger overproduction of thyroid hormone.
The main risk factors for toxic thyroid include being a woman, particularly over the age of 60. Currently, toxic nodules are not thought to be inherited. However, research is being carried out to look at the possibility of genetic involvement in development of toxic nodules (adenomas).
References:
http://www.yourhormones.info/endocrine-conditions/toxic-thyroid-nodule/
http://www.medbroadcast.com/condition/getcondition/thyroid-diseases
https://www.floridahospital.com/toxic-nodular-goiter/causes-toxic-nodular-goiter
