Untreated hypothyroidism may get better or worse over time, depending on its cause and your age.

Hypothyroidism in infants and children

Although rare, hypothyroidism can occur in infants and children. If hypothyroidism is treated within the first month of life, a child will grow and develop normally. Untreated hypothyroidism in infants can cause brain damage, leading to mental retardation and developmental delays. In the United States, all children are tested for hypothyroidism at birth.

Mental retardation usually does not occur if hypothyroidism develops after age 3. But untreated childhood hypothyroidism typically delays physical growth and sexual development, including the onset of puberty. Children may gain weight yet have a slowed growth rate.

Hypothyroidism in adults

Hypothyroidism caused by Hashimoto's thyroiditis occasionally will disappear on its own. More often, the disorder causes a gradual loss of thyroid function, so your symptoms may develop slowly and be so mild that you do not notice them for years. But symptoms usually grow worse, and health problems may develop as the disease continues.

If untreated, hypothyroidism may lead to:

• Myxedema, a condition that causes swelling of tissues, increased fluid around the heart and lungs, slowed muscle reflexes, and a slowed ability to think.
• Myxedema coma, a rare, life-threatening condition. This can occur if you have had hypothyroidism for many years that becomes markedly worse. It usually occurs when older adults who have severe hypothyroidism become ill with another condition, suffer from cold exposure, or take painkillers or sleeping pills. Symptoms include mental deterioration, such as apathy, confusion, and psychosis. You may lose consciousness (coma) and may have an extremely low body temperature (hypothermia), slow heartbeat (fewer than 60 beats per minute), heart failure, and difficulty breathing.
• Complications, such as:
  • Increased levels of cholesterol and triglycerides (increasing the risk of coronary artery disease and stroke).
  • Fluid around the heart (pericardial effusion).
  • Sleep apnea, which causes you to stop breathing for 10 seconds or longer while sleeping.
  • Forgetfulness and dementia.

People with mild (subclinical) hypothyroidism have only slightly abnormal thyroid blood test results and often do not have obvious symptoms or health problems. Some people with mild hypothyroidism regain normal thyroid function, but every year about 2% to 5% of people with subclinical disease develop hypothyroidism; about 20% of women older than 60 have subclinical hypothyroidism.⁴

If your thyroid gland has been removed during surgery, hypothyroidism will occur within a few weeks. If you have been treated with radioactive iodine therapy, hypothyroidism may develop within a year.² In these cases, thyroid function generally does not return, and you have to take thyroid hormone medicine for the rest of your life.

Hypothyroidism during and after pregnancy

Women who have hypothyroidism or mild hypothyroidism before they become pregnant may develop more severe hypothyroidism during their pregnancy. If not treated, pregnant women with hypothyroidism can develop preeclampsia and have a premature delivery. Children born to women with untreated hypothyroidism during pregnancy are at risk for having hypothyroidism at birth and low birth weight and may score lower on intelligence tests than children of healthy mothers.⁵