What is an Underactive Thyroid?

The thyroid gland is a small gland located in the neck. It releases hormones called triiodothyronine (T3) and thyroxine (T4). These hormones are important for the body to function normally.  

An underactive thyroid is when the thyroid gland doesn’t produce enough of these hormones and a person experiences symptoms as a result. The medical name for an underactive thyroid is hypothyroidism.

An underactive thyroid can be diagnosed with a blood test called a thyroid function test. This measures the levels of T4 in your blood and another hormone called thyroid-stimulating hormone (TSH). A high TSH level and a low level of T4 means you may have an underactive thyroid.

Content Reviewed: 15/02/21

Underactive Thyroid Causes

A large majority of cases are due to the thyroid gland being unable to produce enough hormones due to an abnormality with the gland or an iodine deficiency. Sometimes it can be caused by treatments for thyroid cancer or an overactive thyroid. Congenital hypothyroidism occurs when a baby is born with a thyroid gland that isn’t fully developed.

For more information, see the NHS page on hypothyroidism causes.

Underactive Thyroid Treatment

An underactive thyroid has a good prognosis and most people can have their symptoms resolved by taking the medicine Levothyroxine. This is the most common underactive thyroid treatment.  

Levothyroxine is taken daily and the dose varies from person to person. When your doctor first prescribes you Levothyroxine, they will start with a low dose and give you regular blood tests. 

The test results help your doctor find the right dose for you. When you and your doctor find the dose that works for you, the blood tests will become less regular. Most people on a stable dose have a blood test every year to check their thyroid levels.

Levothyroxine should be taken once daily on an empty stomach. Ideally, it should be taken in the morning around 30 minutes before any other medication.

Most of the side effects people can experience with Levothyroxine happen because the dose is too high. This can cause symptoms of an overactive thyroid (hyperthyroidism) such as irritability, tremors, palpitations, and difficulty sleeping. If you notice any of these side effects from Levothyroxine, let your doctor know and they may adjust your dose.

Sometimes people are prescribed combination therapy for an underactive thyroid. This involves treatment with Levothyroxine and T3 together. However, this is not usually prescribed in the UK because there is not enough data to suggest it is better than Levothyroxine alone.

Underactive Thyroid Symptoms

An underactive thyroid can cause a range of symptoms. Some of these symptoms are similar to those of other conditions, making it easy to confuse with other health problems. Some symptoms can develop slowly and people may not realise for a long time that they have symptoms.

Common hypothyroidism symptoms include fatigue, weight gain, sensitivity to cold, constipation, impaired concentration and memory, reduced libido, and muscle aches. It can also cause brittle nails and hair, and dry and scaly skin.