Alternative names for hypothyroidism

Underactive thyroid; myxoedema

What is hypothyroidism? 

Hypothyroidism happens when the thyroid gland, a small, butterfly-shaped gland in your neck, doesn’t make enough thyroid hormones. These hormones help control your body's energy and growth.

How does the thyroid work?

The thyroid gland is like a control centre that works with your brain to release hormones. Your pituitary gland in your brain sends a chemical signal (thyroid stimulating hormone, or TSH) to your thyroid to release hormones (mainly thyroxine, also called T4), which help regulate how your body uses energy. For the thyroid to work well, your body also needs enough iodine, a mineral found in some foods.

What causes hypothyroidism?

• Autoimmune condition: The body mistakenly attacks the thyroid gland, which stops it from working as it should. It does this by producing antibodies that can damage the thyroid gland.
• Other causes: These include surgery to remove the thyroid; destruction of the thyroid after radioactive iodine treatment; lack of iodine; over-treatment of an over-active thyroid; certain medications (like lithium or amiodarone); or very rarely genetic conditions or pituitary disease.
• Congenital: Hypothyroidism can also very rarely be seen at birth (known as congenital hypothyroidism). All newborn babies in the UK are screened for congenital hypothyroidism by a heel prick test a few days after they are born.

What is Hashimoto’s Disease?

This is the autoimmune condition where the body attacks the thyroid with antibodies.  We can measure some of these antibodies with a blood test – the most common are thyroid peroxidase (TPO) antibodies and thyroglobulin antibodies. However just because the body has produced the antibodies it does not mean the thyroid necessarily stops working properly.  So, Hashimoto’s disease is when there are thyroid antibodies in the blood, and thyroid hormone levels can be normal or abnormal.

What about subclinical hypothyroidism?

‘Subclinical’ or ‘mild’ hypothyroidism is sometimes used to describe when the thyroid hormone levels in the blood are within the normal range but the pituitary is having to work harder, and send higher levels of TSH to get the thyroid to work properly. Sometimes, even with the pituitary working harder, the thyroid stops producing enough thyroid hormone, and then ‘overt’ or ‘full’ hypothyroidism develops.

What are the signs and symptoms?

People with hypothyroidism may feel tired, gain weight, feel cold, have dry skin or experience hair loss. Some may have more symptoms than others, depending on how serious the condition is. The symptoms of hypothyroidism are very non-specific, and some surveys suggest that about 1/3 of people with normal thyroid hormone levels have symptoms that are associated with hypothyroidism. This causes confusion and sometimes a wrong belief that a person with otherwise unexplained symptoms must have hypothyroidism; it can also mean that people with hypothyroidism who then start treatment may still have some symptoms.

How common is hypothyroidism?

Hypothyroidism is common, especially in middle-aged women. Around 3.5% of people in the UK have hypothyroidism (Razvi et al., 2019), and 3-10% have subclinical hypothyroidism (Fatourechi, 2009).

Is hypothyroidism inherited?

People with a family history of thyroid problems are more likely to have hypothyroidism. It’s also more common in people with other autoimmune diseases, like type 1 diabetes.

How is hypothyroidism diagnosed?

Doctors diagnose hypothyroidism by testing hormone levels in the blood – they diagnose hypothyroidism if there are high TSH and low T4 levels.

How is hypothyroidism treated?

Hypothyroidism is treated with a medicine called levothyroxine, which replaces the missing thyroid hormones. The aim is to give enough levothyroxine to bring the TSH and T4 levels into the normal reference range, so people with this condition need regular blood tests to make sure their hormone levels stay balanced.

Tablets containing other thyroid hormones, including tri-iodothyronine, or even dried animal thyroid extract do exist. However, there is no good quality research that shows these have any benefit above treatment with levothyroxine alone, so these treatments are not recommended at the moment.

Are there any side-effects to treatment?

Side-effects to levothyroxine treatment are exceptionally rare as levothyroxine is identical to the thyroid hormone naturally produced by the thyroid gland.

How is sub-clinical hypothyroidism treated?

Usually subclinical hypothyroidism is not treated because the T4 levels in the blood are already normal. However, some people with subclinical hypothyroidism have lots of symptoms that could be related to their thyroid, so sometimes they can have a trial of levothyroxine to see if this makes their symptoms better. People with subclinical hypothyroidism who are not on treatment should have their thyroid function checked every year to see if they have developed full hypothyroidism.

Are there any long-term effects?

With treatment, people with hypothyroidism can live normal lives. However, untreated hypothyroidism can cause heart and brain issues.

What about hypothyroidism in pregnancy?

During the first half of pregnancy, the foetus (developing baby) relies on the mother's thyroid hormone (T4) because its own thyroid isn’t fully functioning until around 18–20 weeks. This thyroid hormone is crucial for the baby’s development, especially for the brain.

If a pregnant person has hypothyroidism, they might need a higher dose of levothyroxine to make sure both they and the baby are getting enough of the hormone. In the past, it was suggested that people automatically increase their dose as soon as they get a positive pregnancy test. However, it’s also fine to get a blood test to check thyroid levels early in pregnancy and only increase the dose if needed.

Support for people with hypothyroidism 

British Thyroid Foundation may be able to provide advice and support to patients and their families dealing with hypothyroidism.