What These Hormones Do
The thyroid hormones adjust the metabolism (how fast you "run") by stimulating enzymes that take part in sugar and energy use, temperature regulation, and the burning of calories. The intestinal function (movement and absorption) is fine-tuned for the purpose of altering sugar absorption. They help with the management of fat accumulation or breakdown, which is associated with temperature regulation.
Thyroid hormones affect the cardiovascular system both directly and indirectly. Cholesterol elimination is controlled by thyroid hormones, influencing the tendency toward or away from heart disease. The functional pumping of the heart is directly stimulated by thyroid hormones, resulting in palpitations when there is too much thyroid stimulation (as in hyperthyroidism).
The thyroid controls growth and puberty onset. Thyroid hormones help control bone growth by affecting the breakdown of calcium. They promote the manufacture of proteins, which affect growth and function of the muscles in the body.
Thyroid hormones also take part in the development and functioning of the breasts. Milk production from the breast in the non-pregnant woman ("galactorrhea") should prompt thyroid testing in addition to the other usual tests.
The adrenal gland (which is a whole different story with its own pantheon of hormones) is stimulated by the thyroid, and this can indirectly affect steroid production (like adrenaline).
One of the ways temperature regulation is also controlled is by control of sweat glands. The skin's oil glands are also directed by thyroid hormone, and skin texture can sometimes be a tip-off to a thyroid condition.
Almost a third of patients with depression have thyroid problems, and it's not yet known which causes which. For this reason, depression is another condition that warrants thyroid testing. Normal thyroid function is necessary for normal fetal development of the brain, with both hypothyroidism and hyperthyroidism significant risk factor for intellectual impairment.
Calcitonin is a thyroid hormone that helps maintain the level of calcium in the blood stream (helps deposit it into bone formation, thereby lowering the calcium levels in the blood). This hormone works in concert with the other thyroid effects related to calcium and bone growth.
When Things Go Wrong
Autoimmune disorders, in which antibodies attack the thyroid tissue, can cause increased stimulation as well as destruction of the thyroid (hypothyroidism or hyperthyroidism).
Decreased functioning of the thyroid can be caused by Iodine deficiency (not enough Iodine to make the hormones) or because of destruction of the thyroid gland (radiation or autoimmune disease). Being from the New Orleans area, I have Iodine comin' out my ears, thanks to the seafood in my diet. But there are places where supplemental Iodine needs to be considered. Autoimmune disease can produce an inflammation ("thyroiditis") that can limit function. Removal of the thyroid, necessary to remove thyroid tumors, can remove enough functioning thyroid to cause hypothyroidism, too.
In pregnancy, there is increased excretion of Iodine, plus the fetus sifts a lot of it out of the maternal circulation, so that if a woman has an Iodine deficiency to begin with, there is an increased danger of hypothyroidism. Usually prenatal vitamins can solve this dietary problem.
Symptoms of hypothyroidism are lethargy, cold intolerance, weight gain, and hair and skin changes. In its most severe form, called "myxedema," there is excessive sedation, abnormally low sodium levels, respiratory distress, and possibly death. Myxedema is rare, usually seen in undiagnosed elderly patients, making pregnancy myxedema a non-issue.
Treatment of hypothyroidism is usually by way of supplementing a patient with T4 (e.g., Synthroid).
Out of control or undiagnosed hypothyroidism can complicate pregnancy by causing cretinism (growth and brain abnormalities) in the fetus if the maternal hypothyroidism is severe enough to be associated with Iodine deficiency in the fetus, too. But proper control of hypothyroidism should be plenty good enough to prevent these types of problems. Treatment is based on the cause, treated with either a prescription of levothyroxine (T4, as the drug Synthroid) or Iodine supplementation, or both. Synthroid is FDA Drug Class B, so it's considered safe, especially since not treating hypothyroidism carries the risks above.