Simply put, the thyroid gland is like a governor on a motor–-it sets the speed of metabolism for your body. If it sets that metabolism too high, you can burn up your motor, so to speak. Weight loss, rapid heart rate, and a host of other unpleasant side effects can alter the way you feel with a higher than normal metabolism. In fact, when morning sickness of pregnancy increases in intensity into the more ugly, "hyperemesis," an abnormality of the thyroid function should be suspected.
"Euthyroid" is a medical slang word meaning everything is just fine with this gland. "Hyperthyroid" means increased functioning into the abnormal range as described above. "Hypothyroid" means decreased function and results in weight gain (you don't burn off calories), lethargy and fatigue, and possibly goiter formation, which is an enlargement of the lackluster thyroid trying to overgrow to make up for the decreased function.
Like most glands, there are hormones related to the thyroid and its function. The thyroid cranks out mainly T3 and T4 (tri-iodothyronine and thyroxine, respectively), and calcitonin. The T3 and T4 are what are called the thyroid hormones, and they differ from each other in that the T3 is more powerful, while the T4 is more plentiful. Once in the blood stream, T4 is converted into T3. Calcitonin helps manage calcium levels in the body, which is important in bone health since so much of bone is calcium.
The Hypothalamic-Pituitary-Thyroid Axis
TSH, "thyroid stimulating hormone," is made by the pituitary gland. The pituitary gland gets signals from another gland even higher up in the pecking order, the hypothalamus. The ultimate output of T3 and T4 from the thyroid gland is based on readings of what's in the circulation already, such that hypofunction (under-function) prompts the stimulating hormones to prod some extra action from the thyroid, and hyperfunction (over-function) results in a blunting of the output of these stimulating hormones. This whole back-and-forth relationship is like a thermostat, and is used throughout the body on many levels. (For instance, the entire menstrual cycle works like this via a hypothalamus-pituitary-ovarian axis.)
The Thyroid Hormones
Properly stimulated by the pituitary and hypothalamus (the vice president and the CEO of the body, respectively), the thyroid cranks out the T3 and T4 as needed. These thyroid hormones are bound to proteins, which render them inactive until they're freed–-then they can do their job.
Both are made using Iodine, so it's important to supplement the diet (iodized salt, for instance) with Iodine in areas where it isn't plentiful.