Hypothyroidism (underactive thyroid): symptoms, causes and tests

The thyroid is a small gland at the front of your neck, and its hormones help control how your body uses energy. When it makes too little, many body functions slow down. This is called hypothyroidism, or an underactive thyroid. This article explains the basics based on the NIDDK and NHS. It is general education, not a diagnosis.

What is hypothyroidism?

The NIDDK explains that hypothyroidism occurs when the thyroid gland does not make enough hormone for the body's needs. Because thyroid hormones regulate how the body uses energy, a shortage slows many body functions. The NIDDK says about 5 in 100 Americans aged 12 and older have it, usually mildly.

Common symptoms

The NIDDK lists fatigue, weight gain, trouble tolerating cold, joint and muscle pain, dry skin, thinning hair, heavy or irregular periods or fertility problems, a slowed heart rate and depression. The NHS adds constipation, poor concentration, low mood and a hoarse voice. Symptoms usually develop slowly and may be mild. The NIDDK says they can go unnoticed for months or years, and that fatigue and weight gain are common and do not necessarily point to the thyroid.

Causes

The NIDDK says the most common cause is Hashimoto's disease, an autoimmune disorder. The NHS agrees that this is the main cause in the UK. Other causes include:

• Thyroiditis, which is inflammation of the thyroid
• Congenital hypothyroidism, which is present at birth
• Thyroid surgery or radiation treatment
• Certain medicines
• Less often, iodine imbalance or problems with the pituitary gland or hypothalamus

The NIDDK says iodine deficiency is extremely rare in the United States, and the NHS says it is rare in the UK.

Who is more at risk?

The NIDDK says women and people over 60 are more likely to develop hypothyroidism. Risk is higher with a past thyroid problem, thyroid surgery, radiation to the neck or chest, a family history of thyroid disease, pregnancy in the past 6 months, or Turner syndrome. Conditions linked with it include celiac disease, Sjögren's syndrome, pernicious anaemia, diabetes, rheumatoid arthritis and lupus.

How is it diagnosed?

The NIDDK says doctors take a medical history, do a physical exam and order thyroid blood tests, and sometimes imaging. Symptoms alone cannot confirm the diagnosis because they overlap with many other conditions. The NHS says blood tests measure thyroid-stimulating hormone (TSH) and thyroxine (T4), and a GP may refer you to a specialist for further tests.

How is it treated?

The NIDDK says hypothyroidism is treated by replacing the missing hormone with a thyroid hormone medicine, which is the same as the hormone a healthy thyroid makes. The NHS says most people take it for life, with regular blood tests to check that the dose is right. A wrong dose can cause side effects, the NHS says, and the NIDDK warns against stopping the medicine without talking to a doctor. Your doctor decides the right medicine and amount for you.

Complications

According to the NIDDK, hypothyroidism can contribute to high cholesterol. Rarely, severe untreated hypothyroidism can lead to myxedema coma, a life-threatening state in which body functions slow dangerously. It needs immediate treatment.

Pregnancy

The NIDDK says untreated hypothyroidism can affect both mother and baby, and that thyroid medicines can prevent these problems and are safe during pregnancy. The NHS says to tell your GP if you are pregnant or planning to be, because you will need more tests and your dose is likely to need adjusting. See our article on thyroid problems in pregnancy for more detail.

When to see a doctor

See a doctor if you think you may have an underactive thyroid, because many symptoms have other causes and need to be checked. The NIDDK also advises getting tested if you have high cholesterol, and contacting your doctor right away if you become pregnant while having hypothyroidism.

Seek emergency help for extreme drowsiness, confusion, very slow breathing, or a very low body temperature, which can signal myxedema coma. In an emergency, call your local emergency number (112 in India, 911 in the United States).

This article is for education only and does not replace advice from your own doctor.