The thyroid is a gland located in the neck below the Adam's apple. It helps control the body's metabolic rate by producing the hormones thyroxine (T4) and triiodothyronine (T3). A metabolic rate is the rate of chemical processes occurring within the body that are necessary to maintain life.
Hypothyroidism is the most common of the thyroid disorders. It occurs when the thyroid gland becomes underactive and does not produce enough thyroid hormones. The metabolic rate falls and normal bodily functions slow down.
Hypothyroidism occurs in 1.5% to 2% of women and in 0.2% of men, and it is more common with increasing age. Up to 10% of women over the age of 65 show some signs of hypothyroidism.
Although less common, hypothyroidism does occur among the young. Neonatal hypothyroidism, called cretinism, is associated with mental retardation, jaundice (yellowing of skin), poor feeding, breathing difficulties, and growth problems. Childhood (juvenile) hypothyroidism is characterized by delayed growth and problems with mental development; however, with prompt treatment, problems can be minimized.
Primary hypothyroidism occurs when there is a problem with the thyroid gland itself. The most common cause of adult hypothyroidism is Hashimoto's thyroiditis. It's caused by an autoimmune process where the body produces antibodies that attack and gradually destroy the thyroid gland.
Women are eight times more likely than men to develop Hashimoto's thyroiditis, especially as they age. It can also run in families or be associated with syndromes of genetic abnormalities such as Turner's syndrome, Klinefelter's syndrome, and Down syndrome.
Hypothyroidism can also be caused by treatments for hyperthyroidism. To treat hyperthyroidism, the thyroid gland may be rendered inactive with medications or radioactive iodine treatment, or it may be surgically removed. The result may be a lack of thyroid hormones, causing hypothyroidism.
The thyroid gland requires iodine to function properly. A chronic lack of iodine means that less thyroid hormone can be produced and this causes the thyroid to enlarge. Since salt manufacturers now add iodine to salt, this form of hypothyroidism is extremely rare in North America. However, it is still the major cause of hypothyroidism in underdeveloped countries, where iodine is often lacking in the diet.
Some rare inherited disorders cause enzyme abnormalities in the thyroid gland that don't allow it to make the hormones. Secondary hypothyroidism (when there is a problem with the pituitary or hypothalamus, not the thyroid) may occur if the pituitary gland in the brain isn't working properly, and not producing a hormone (called thyroid-stimulating hormone (TSH)) needed to stimulate the thyroid gland. These disorders are quite rare and not a major cause of hypothyroidism.
Some medications can cause a person to develop hypothyroidism by interfering with the production of the thyroid hormone. These include amiodarone* (a heart medication), lithium (a bipolar disorder medication), and interferon alpha (a cancer medication).
Symptoms and Complications
Because the hormones T4 and T3 are lacking, the body's metabolic rate and organ functions slow down. Symptoms aren't always obvious, and hypothyroidism may sometimes be mistaken for other diseases.
People with hypothyroidism often experience:
- coarse and dry hair
- cold intolerance
- confusion or forgetfulness (often mistaken for dementia in seniors)
- dry, scaly skin
- fatigue, or feeling sluggish
- hair loss
- increased menstrual flow (women)
- muscle cramps
- slower heart rate
- weight gain
It is important to note that these symptoms tend to be more subtle in seniors. Seniors may experience symptoms of confusion, anorexia, weight loss, falling, and decreased mobility.
If hypothyroidism isn't treated, the symptoms will progress and, rarely, a severe form of hypothyroidism called myxedema will develop. With myxedema, the body temperature drops, anemia occurs, and there is a risk of congestive heart failure. Myxedema coma occurs in some people with severe hypothyroidism who are exposed to some kind of stress, such as an infection, severe cold, trauma, or the use of sedatives. Symptoms include loss of consciousness, seizures, slowed breathing, and decreased mental activity.
Making the Diagnosis
If your doctor suspects hypothyroidism, tests can be performed to check how well the thyroid gland is functioning. Levels of T4 in the blood are measured, along with thyroid-stimulating hormone (TSH) that is produced by the pituitary gland. High levels of TSH suggest that the thyroid is underactive and that the pituitary is overcompensating by making excess hormones to stimulate the thyroid gland.
With secondary hypothyroidism, the pituitary gland isn't functioning properly and T4 levels in the blood are low. In this case, TSH levels can be low or inappropriately normal, meaning that levels are normal when a doctor wouldn’t expect them to be normal based on a person’s medical condition.
Thyroid scans, using radioactive iodine or technetium, allow doctors to see the thyroid gland and identify areas that are less active. Thyroid ultrasound scans provide a detailed anatomic imaging of the gland, and can identify lesions (e.g. nodules, cysts), provide accurate size measurement, and show blood flow properties throughout the gland.
Treatment and Prevention
Treating hypothyroidism requires replacing hormones with synthetic or natural thyroid hormones. The easiest and most effective thyroid hormone replacement involves a synthetic form of T4 that is taken orally (by mouth). The tablets are available in a variety of strengths, and doses are adjusted for each individual until the correct hormone levels in the blood are reached and there is a satisfactory control of symptoms.
So-called "natural" thyroid supplements, which usually contain ground-up thyroid glands from animals, are rarely used now. This is because they can cause allergic reactions and do not provide a consistent dose of T4.
Large doses of T4 can have serious side effects. Age- and body weight-appropriate doses are therefore given at first and slowly increased until levels of T4 and TSH in the blood are back to normal.
People with mild hypothyroidism might feel fine and not require treatment, while others will be bothered by symptoms and require treatment. Hypothyroidism tends to get worse over time and even mild cases should be regularly monitored. Individuals taking small amounts of thyroid hormones may need to have their doses increased with time. Beyond a certain age, seniors sometimes need lower doses of T4.
If a dysfunctional pituitary gland is the cause of hypothyroidism, it will need to be treated. The pituitary controls many other glands in the body as well, and several medications may be needed if a person has other pituitary hormonal disorders.
*All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.