Low thyroid levels can cause many different symptoms, including:

• Feeling tired, weak, or depressed.
• Dry skin and brittle nails.
• Not being able to stand the cold.
• Constipation.
• Memory problems or trouble thinking clearly.
• Heavy or irregular menstrual periods.

Symptoms occur slowly over time. At first you might not notice them, or you might mistake them for normal aging.

Symptoms in infants, children, teens, and pregnant women

Although rare, hypothyroidism can occur in infants, children, and teens. In infants, symptoms include a poor appetite and choking on food. Symptoms may also include dry, scaly skin. In children and teens, symptoms include behaviour problems and changes in school performance. Children and teens may gain weight but have a slowed growth rate. Teens may have delayed puberty and look much younger than their age.

Symptoms of hypothyroidism during and after pregnancy include fatigue, weight loss, dizziness, depression, and memory and concentration problems.

Hypothyroidism caused by Hashimoto's thyroiditis sometimes goes away on its own. More often it causes gradual loss of thyroid function. Symptoms may develop slowly and be mild. But symptoms usually grow worse, and health problems may develop over time.

If untreated, hypothyroidism may lead to myxedema. This condition causes swelling of tissues, increased fluid around the heart and lungs, slowed muscle reflexes, and a slowed ability to think. In rare cases, it can cause a coma which can be life-threatening.

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 who have mild hypothyroidism regain normal thyroid function.

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. In these cases, thyroid function typically doesn't return.

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 intellectual disability and developmental delays. In Canada, all children are tested for hypothyroidism at birth.

Intellectual disability 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.

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Call 911 or other emergency services immediately if you or a person you know has hypothyroidism and has signs of myxedema coma, such as:

• Mental deterioration, such as apathy, confusion, or psychosis.
• Extreme weakness and fatigue that progress to loss of consciousness (coma).
• Severe trouble breathing, slow heart rate (less than 60 beats per minute), or low body temperature [35°C (95°F) or below].

See your doctor if you have any symptoms that don't go away, including:

• Depression.
• Feeling tired, sluggish, or weak.
• Memory problems or trouble concentrating.
• An inability to tolerate cold.
• Dry skin, brittle nails, or a yellowish tint to the skin.
• Constipation.
• Heavy or irregular menstrual periods that may last longer than 5 to 7 days.

If you have one or two of the above symptoms that have not changed or have changed very little over a long period of time, it's less likely that the symptoms are caused by hypothyroidism. Consult your doctor.

Talk to a doctor if you are pregnant and have some of the above symptoms. Also talk to a doctor if you have hypothyroidism and are pregnant or are trying to become pregnant. Your dose of thyroid hormone medicine may need to be changed.

Watchful waiting

Watchful waiting is a wait-and-see approach. It's not a good choice for hypothyroidism that is causing symptoms. Treatment should start as soon as the condition is diagnosed.

Watchful waiting may be okay for certain adults with mild (subclinical) hypothyroidism whose blood tests show only small changes. Talk to your doctor about treatment and possible risks and benefits. Watch for any signs that your hypothyroidism is getting worse. Doctors often want people to have yearly thyroid function blood tests to check to see if thyroid hormone production is normal.

To diagnose hypothyroidism, your doctor will first ask you about your past health problems and do a physical exam. If your doctor thinks you have the condition, a simple blood test can show if your thyroid hormone level is too low. The blood tests used most often are:

• Thyroid-stimulating hormone (TSH) test.
• Thyroxine (T4) measurement.

If the results of the above tests aren't normal, you may have antithyroid antibody tests. These tests can tell if you have the autoimmune disease Hashimoto's thyroiditis. When you have this disease, your body's defence system attacks the thyroid gland.

Because hypothyroidism may cause problems with thinking and learning, most provinces test newborns for the condition. If your baby wasn't born in a hospital or if you think your baby may not have been tested, talk to your doctor.

Screening for hypothyroidism

The Canadian Task Force on Preventive Health Care does not recommend screening for hypothyroidism in adults who do not have symptoms. If you are pregnant or have had thyroid problems, talk to your doctor about screening.^{footnote 1}

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Thyroid hormone medicine is the only way to treat hypothyroidism. Your doctor will treat your low thyroid level with the thyroid pills levothyroxine. Usually, thyroid hormone medicine:

• Reduces or gets rid of symptoms of hypothyroidism. Symptoms often improve within the first week after you begin therapy. All symptoms often go away within a few months.
• May reduce the risk of slowed physical growth, intellectual disability, and problems with behaviour in infants and children.

After you start treatment, you'll have regular visits with your doctor to see if you have the right dose of medicine. Getting too much or too little thyroid hormone can cause problems.

Thyroid hormone medicine does not cause side effects if you take the correct dose. Depending on its cause, people who have hypothyroidism may need treatment for the rest of their lives.

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• Take your thyroid hormone medicine exactly as prescribed. Call your doctor or nurse advice line if you think you are having a problem with your medicine. Most people do not have side effects if they take the right amount of medicine regularly.
  • Take the medicine 30 minutes before breakfast, and do not take it with calcium, vitamins, or iron.
  • Do not take extra doses of your thyroid medicine. It will not help you get better any faster, and it may cause side effects.
  • If you forget to take a dose, do NOT take a double dose of medicine. Take your usual dose the next day.
• Tell your doctor about all prescription, herbal, or over-the-counter products you take.
• Take care of yourself. Eat a healthy diet, get enough sleep, and get regular exercise.

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