Abnormally increased functioning of the thyroid is usually due to Graves' Disease. In this autoimmune disorder, there is over-stimulation, resulting in hyperthyroidism. Another cause can be an over-functioning "hot spot" called a toxic nodule (of thyroid tissue). And if you really want to get into some weird stuff, there can be thyroid tissue in a type of tumor of the ovary (dermoid cyst), which can be functional enough to cause hyperthyroidism ("struma ovarii"). I've even had a case of thyroid cancer arising from such ovarian tissue, but this is extremely rare.
Symptoms of hyperthyroidism include restlessness, fast pulse, intolerance to heat, and weight loss. The bulging eyes, called "exopthalmos," are the most striking physical characteristic. How bad can it get? In a word ... very. "Thyroid storm" is a name that's as bad as it sounds. It can result in fever, rapid pulse to the point of atrial fibrillation, shock, confusion, psychosis, seizures, coma, and death.
In pregnancy, hyperthyroidism can aggravate morning sickness into the much more debilitating hyperemesis gravidarum, requiring hospitalization for rehydration. (In my practice, anyone with morning sickness is evaluated for thyroid function again after the initial labs.) But this just covers the mother. Hyperthyroidism can impact the pregnancy, too, with increased risk toward preterm labor, premature deliveries, low birth weight infants, and pregnancy induced hypertension (pre-eclampsia). The antibodies that make the mother's thyroid overreact can pass through the placenta to the baby, causing hyperthyroidism in the fetus, too.
Because pregnancy tones down a woman's immune response to everything (so that she won't reject her baby), immune disease-caused hyperthyroidism may cool down after a temporary surge of activity at 15 weeks, but the medical community isn't in agreement about the studies on this area. However, based on this theory, a doctor should be wary of "thyrotoxicosis" in the early to mid second trimester as well.
Treatment of hyperthyroidism in pregnancy is a little more challenging than treating hypothyroidism (where merely supplementing T4 may be enough). Medicines to tone down thyroid function are a bit more artificial than the more natural T4-like Synthroid. Since these are chemically constructed drugs, prescribing them may walk a patient down the FDA letters of risk (T4 is only "B," little if any risk).
PTU (propylthiouracil) is the time-honored therapy. This drug works by interfering with the thyroid gland's use of Iodine to make thyroid hormones. Unfortunately, PTU crosses the placenta to the baby and can cause a mild hypothyroid condition–-even goiters-–in a minority of newborns. For this reason, it's FDA risk letter is "D." The risk to the fetus is late in pregnancy, because the fetal thyroid does not begin making thyroid hormones until the end of the first trimester. Still, it's considered the safest approach to hyperthyroidism. Not treating hyperthyroidism is even riskier.
Steroids are used to help control thyroid storm and have a significant safety margin in pregnancy. Regardless, thyroid storm is extremely dangerous and can't go untreated.
Thyroid Confusion: Hyperemesis Gravidarum
Hyperemesis gravidarum is the abnormal distortion to the more common morning sickness. Weight loss is never normal in pregnancy, and nutrition is crucial during that first trimester when major developmental trends are happening in the fetus. So when the nausea and vomiting progress into an unstoppable starvation, hospitalization is necessary.
At a molecular level, there is a similarity of the pregnancy hormone hCG to the thyroid hormone. They both sport an identical component–the "alpha chain molecule." (It's the "beta-chain molecule" on each that makes them the different hormones that they are.) Unfortunately, the rise in hCG that comes with pregnancy, and with it a rise in an alpha-chain molecule, makes the body react like there's too much thyroid hormone, aggravating a patient into a morning sickness or even hyperemesis. No one knows if the alpha-chain of hCG, making the body act like there's too much thyroid hormone, is THE reason for morning sickness, but certainly there's some input from this phenomenon. In the absence of actual hyperthyroidism, hyperemesis is treated with hydration and anti-nausea medications, not with medicines to treat hyperthyroidism.
In summary, autoimmune diseases can make the thyroid hyperfunction or hypofunction. Thyroid hormone supplementation treats "hypo," and "hyper" is treated with drugs like PTU that interfere with using iodine to make thyroid hormones. Both conditions are dangerous to mother, baby, and the pregnancy on the whole if untreated.