The thyroid is a small gland located below the Adam's apple in your neck. It releases hormones, thyroxine (T4) and triiodothyronine (T3), which increase the amount of oxygen your body uses and stimulate your cells to produce new proteins. By controlling the release of these hormones, the thyroid determines the metabolic rate of most of your body's organs.
The thyroid gland is regulated by thyroid-stimulating hormone (TSH), which is made by the pituitary gland in the brain. Normally, when thyroid hormone levels in the body are high, they will "switch off" the production of TSH, which in turn stops the thyroid from making more T4 and T3.
Problems occur when the thyroid gland becomes either underactive (hypothyroidism) or overactive (hyperthyroidism). Thyroid problems are more common in women than men. Cancer may also develop in the thyroid gland.
Thyroid diseases sometimes result from inappropriate TSH levels, or may be caused by problems in the thyroid gland itself.
The most common cause of hypothyroidism is Hashimoto's thyroiditis,an autoimmune condition where the body makes antibodies that destroy parts of the thyroid gland. Surgical removal and certain medications (e.g., amiodarone, lithium) can also cause hypothyroidism. Treatment for hyperthyroidism can cause hypothyroidism as well.
Other causes of hypothyroidism include pituitary problems, hypothalamus problems, and iodine deficiency (rare in North America, but affects nearly 2 billion people worldwide). Some babies are born with hypothyroidism – this is called congenital hypothyroidism.
There are different causes of hyperthyroidism. Graves' disease is the most common cause of hyperthyroidism. This condition occurs when the immune system produces an antibody that stimulates the entire thyroid gland; this leads to overactivity and higher levels of thyroid hormones.
Another form of hyperthyroidism is called toxic nodular goiter or toxic thyroid adenoma. Adenomas, abnormal nodules of tissue in the thyroid, constantly produce thyroid hormones even when they are not needed.
Secondary hyperthyroidism is caused when the pituitary gland makes too much TSH, leading to constant stimulation of the thyroid gland. A pituitary tumour may cause TSH levels to rise. More rarely, the pituitary gland becomes insensitive to thyroid hormones, no longer responding to high levels.
Another possible cause of hyperthyroidism is a condition called thyroiditis. This condition occurs when the thyroid gland becomes inflamed. Depending on the type of thyroiditis, this may lead to temporary hyperthyroidism that might be followed by hypothyroidism.
There are four types of thyroid cancers: papillary, follicular, anaplastic, and medullary cancer. These are associated with radiation treatment to the head, neck, or chest. Radiation treatment for benign (non-cancerous) conditions is no longer carried out in these areas, but was more common in the past. In other cases, a genetic mutation might be associated with thyroid cancer, either alone or in conjunction with other types of cancers (e.g., multiple endocrine neoplasia, BRAF gene mutations). Less commonly, other cancers might metastasize to the thyroid (e.g., lymphoma, breast cancer).
Symptoms and Complications
Hypothyroidism results in low levels of T4 and T3 in the blood. Not having enough T4 and T3 in the blood causes your metabolism to slow down.
Common symptoms include:
- coarse and dry hair
- confusion or forgetfulness (often mistaken for dementia in seniors)
- droopy eyelids
- dry, scaly skin
- fatigue or a feeling of sluggishness
- facial puffiness
- hair loss
- hoarse speech
- increased menstrual flow (women)
- intolerance to cold temperatures
- muscle cramps
- slower heart rate
- weight gain
If hypothyroidism isn't treated, the symptoms will progress. Rarely, a severe form of hypothyroidism, called myxedema, can develop. Symptoms of myxedema include:
- low body temperature
- dulled mental processes
- congestive heart failure, a condition where the heart cannot pump enough blood to meet the body's needs
Myxedema coma occurs in people with severe hypothyroidism who have been exposed to additional physical stresses such as infections, cold temperatures, trauma, or the use of sedatives. Symptoms include loss of consciousness, seizures, and slowed breathing.
Hyperthyroidism results in high levels of T4 and T3 circulating in the blood. These hormones speed up your metabolism. Some of the most common symptoms include:
- increased heart rate with abnormal rhythm or pounding (palpitations)
- high blood pressure
- increased body temperature (feeling unusually warm)
- increased sweating
- feeling agitated or nervous
- tremors in the hands
- feeling of restlessness even though the person is tired or weak
- increased appetite accompanied by weight loss
- interrupted sleep
- frequent bowel movements, sometimes with diarrhea
- puffiness around the eyes, increased tears, sensitivity to light, or an intense stare
- bone loss (osteoporosis)
- stopped menstrual cycles
Graves' disease, in addition to the common symptoms of hyperthyroidism, may cause a bulge in the neck (goiter) at the location of the enlarged thyroid gland. It also might cause the eyes to bulge out, which may result in double vision. Sometimes, the skin over the shins becomes raised.
If hyperthyroidism is left untreated or is not treated properly, a life-threatening complication called thyroid storm (extreme overactivity of the thyroid gland) can occur. Symptoms include:
- high blood pressure
- irregular heartbeat, which can be fatal
- jaundice associated with liver enlargement
- mood swings
- muscle wasting
Thyroid storm, considered a medical emergency, can also be triggered by trauma, infection, surgery, uncontrolled diabetes, pregnancy or labour, or taking too much thyroid medication.
Making the Diagnosis
Thyroid disease suspected by clinical history and physical exam is confirmed by laboratory tests. Laboratory tests usually measure levels of TSH and thyroid hormones. Serology tests can measure the levels of antibodies associated with hypothyroidism and hyperthyroidism. If your doctor suspects thyroid cancer, a biopsy can be used to sample the thyroid tissue and test for cancer.
Another method called a functional stimulation test can be used to distinguish whether the pituitary and thyroid glands are the source of medical symptoms. Ultrasounds and nuclear thyroid scans allow for visual and functional examination of the thyroid gland or of nodules.
Treatment and Prevention
The usual treatment for hypothyroidism is thyroid hormone replacement therapy. With this treatment, synthetic thyroid hormone (e.g., levothyroxine*) is taken by mouth to replace the missing thyroid hormone. Treatment is usually life-long.
Most people who take thyroid replacement therapy do not experience side effects. However, if too much thyroid hormone is taken, symptoms can include shakiness, heart palpitations, and difficulty sleeping. Women who are pregnant may require an increase in their thyroid replacement by up to 50%. It takes about 4 to 6 weeks for the effect of an initial dose or change in dose to be reflected in laboratory tests.
Hyperthyroidism can be treated with iodine (including radioactive iodine), anti-thyroid medications or surgery.
Radioactive iodine can destroy parts of the thyroid gland. This may be enough to get hyperthyroidism under control. In at least 80% of cases, one dose of radioactive iodine is able to cure hyperthyroidism. However, if too much of the thyroid is destroyed, the result is hypothyroidism. Radioactive iodine is used at low enough levels so that no damage is caused to the rest of the body. It isn't given to pregnant women because it may destroy the thyroid gland of the developing fetus.
Larger doses of regular iodine, which does not destroy the thyroid gland, help block the release of thyroid hormones. It is used for the emergency treatment of thyroid storm, and to reduce the excess production of thyroid hormones before surgery.
Anti-thyroid medications (e.g., propylthiouracil* or methimazole) can bring hyperthyroidism under control within 6 weeks to 3 months. These medications cause a decrease in the production of new thyroid hormones by the thyroid gland. Larger doses will work more quickly, but may cause side effects including skin rashes, nausea, loss of taste sensation, liver cell injury, and, rarely, a decrease of blood cell production in the bone marrow.
Surgical removal of the thyroid gland, called thyroidectomy, is sometimes necessary. It may be required if there are cancerous nodules; if a non-cancerous nodule is causing problems breathing or swallowing; if the person cannot take radioactive iodine or antithyroid medications, or if these do not work; or if a nodule that contains fluid continues to cause problems. Removing the thyroid gland leads to hypothyroidism, which must then be treated with thyroid hormone therapy for the rest of a person's life.
Sometimes your doctor may recommend other medications to help control symptoms of hyperthyroidism, such as shakiness, increased heart rate, anxiety, and nervousness. However, these won't cure thyroid dysfunction.
Treatment for thyroid cancers often involves some combination of thyroidectomy (surgical removal of the thyroid gland), radioactive iodine, radiation therapy (less common), anticancer medications, and hormone suppression.