In layman's terms please...

[MitziAnne]

What exactly does the thyroid do? What happens to make it stop working properly? Is it ever 'fixed'?

[PS Diva]

The thyroid is a gland that produces hormones which help regulate metabolism and growth. Whether or not it can be "fixed" depends on what is wrong with it. There are several problems one could have...

[LisaS]

the thyroid's basic job is to make the hormone T3 and T4 (two hormones whose real names are tongue-twisters so everyone calls them T3 and T4). As the other poster said, the primary effects of these hormones are on your basal metabolism.

nearly every cell in the body has a receptor for T3. T3, like many other hormones, once transported into the nucleus of the cell, can influence gene transcription by its presence and/or absence. Then the resulting proteins that are either made or not made effect how that cell works. Other cellular organelles can also bind T3 and change how they work by its presence or absence.

Here is a quick overview of the thyroid with good related links:
http://www.endocrineweb.com/thyfunction.html

here is a semi-technical article about the thyroid - skip down to Cellular effects of T3
http://www.anaesthetist.com/icu/organs/endocr/thyroid/thyfx.htm

[Nancy LC]

It actually makes more hormones than those 2. It also makes T1-T7 and Calcitonin, which is useful for building bones.

The thing that makes it malfunction... well, autoimmune disease is probably the biggest and there seems to be a connection between diet and autoimmune disease. Grains and dairy seem to be possible offenders.

But also, the thyroid can be fine but the pituitary gland (in the brain) can malfunction and cause hormones that signal the thyroid to be too much or too little. That in turn can lead a perfectly fine thyroid gland to doing the wrong thing.