A multinodular goiter is simply a late-stage goiter, meaning a goiter that’s been around for a while and has had a chance to grow and become lumpy. “Goiter” simply means “enlarged thyroid” – so you can technically use the term any time a patient has a big thyroid (like, for example, patients with Graves disease). In this context, however, with the word multinodular attached, it means a goiter that is caused by an inability to produce thyroid hormone (which can happen for many reasons; in underdeveloped countries it’s commonly because of iodine deficiency, whereas in this country the reasons are often unclear). A low T4 level causes the pituitary to secrete more TSH, which makes the thyroid grow bigger (hence, the goiter). As this process evolves, some areas of the thyroid are growing and trying to produce thyroid hormone and others are not growing, but involuting. Repeated cycles of growth and involution can damage the thyroid, and as it repairs itself, fibrosis occurs, leading to a lumpy, nodular appearance in the goiter.”Toxic” means that the thyroid is producing thyroid hormone at an increased rate. The term toxic multinodular goiter is loosely used to describe any lumpy thyroid that is producing excess thyroid hormone. This could occur in a thyroid adenoma that’s producing thyroid hormone (and which appears clinically as a lump) or it could occur within the context of a true multinodular goiter. Sometimes, for reasons that are not clear, a nodule within a multinodular goiter can sort of break away and start producing thyroid hormone autonomously (without the input of TSH). In these instances, the patient may actually become hyperthyroid!Note: the cool vintage Morton Salt ad belongs to jbcurio, and can be found at: http://www.flickr.com/photos/jbcurio/2173554959/.