Q. What is the difference between a thyroid nodule, a multinodular goiter and a toxic multinodular goiter?

A. Great question! Let’s look at each separately.

Thyroid nodule

A thyroid nodule is simply what it sounds like: a lump in the thyroid, usually one that’s felt by the patient or the clinician. The term “thyroid nodule” doesn’t indicate anything about the underlying pathology, it is simply a clinical, descriptive term.

Lumps or nodules in the thyroid can be caused by many different things, including both non-neoplastic disorders (multinodular goiter, thyroiditis, Graves disease), and neoplasms (adenoma and carcinoma). Non-neoplastic disorders are the most common cause, followed by adenoma, and then carcinoma.

So nodules should always be investigated because of the possibility (though small) of carcinoma.

Multinodular goiter

A multinodular goiter is simply 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 technically, you can use the term goiter to refer to any big thyroid.

But when you put the term multinodular in front of goiter, it usually means that the goiter is caused by an inability to produce thyroid hormone at a normal rate. There are lots of reasons a patient may be unable to make thyroid hormone.

In underdeveloped countries multinodular goiters most commonly are a result of iodine deficiency. This really bugs me, because how hard would it be to just give people in these areas iodinized salt? Not hard at all. So why don’t we do this?

In this country, iodine deficiency is rare (check out the photo above of iodinized salt), and the reasons for decreased thyroid hormone production are often unclear.

Either way, 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-bumpy, multinodular goiter.

Toxic multinodular goiter

“Toxic” means that the thyroid is producing thyroid hormone at a greatly 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 (caused by the inability to make thyroid hormone).

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://w.com/photos/jbcurio/2173554959/.