Underactive and overactive thyroid compared: symptoms, causes and treatment

What does the thyroid do?

The thyroid is a gland in the neck that makes hormones which help control the body's energy use. An underactive thyroid does not make enough of these hormones, while an overactive thyroid makes too much.

What are the symptoms of an underactive thyroid?

The NHS describes an underactive thyroid as a common condition in which the thyroid does not make enough hormone. Common symptoms include tiredness, feeling colder than usual, weight gain, constipation, low mood, and dry skin or hair.

Symptoms may be mild and can develop slowly, so they are not always easy to notice.

Because the symptoms of an underactive thyroid overlap with other conditions, it cannot be diagnosed from symptoms alone.

What are the symptoms of an overactive thyroid?

The NHS says an overactive thyroid, also called hyperthyroidism, means the thyroid makes too much hormone. Symptoms can include nervousness, anxiety, mood swings, sleep problems, tiredness, sensitivity to heat, and a fast or irregular heartbeat.

Tremors and weight loss can also occur, and it is unlikely that one person would have all of the symptoms.

What causes each condition?

In the UK, the main cause of an underactive thyroid is Hashimoto's disease, an autoimmune condition in which the immune system attacks the thyroid.

The main cause of an overactive thyroid is Graves' disease, another autoimmune condition. Thyroid nodules can also cause an overactive thyroid, though less commonly.

Other causes of an underactive thyroid include thyroiditis, neck surgery or radiotherapy, treatment for an overactive thyroid, and some medicines.

How are they treated?

An underactive thyroid is usually treated with levothyroxine tablets, which replace the missing hormone. The NHS says they are usually needed for life, with regular blood tests to check that the treatment is right.

An overactive thyroid can be treated with antithyroid medicine, radioactive iodine or surgery. Beta blockers can help control some of the symptoms in the meantime, but they do not stop the thyroid making too much hormone.

Radioactive iodine often leads to an underactive thyroid that then needs lifelong hormone tablets.

How is the thyroid tested, and when should someone get help?

Thyroid function is checked with blood tests that measure thyroid-stimulating hormone (TSH) and thyroid hormones. The site's pages on thyroid function tests and TSH explain them, and the laboratory report shows the reference range used.

A GP can examine the neck and arrange blood tests to check how well the thyroid is working. Thyroid storm is a rare, life-threatening flare-up of an overactive thyroid that the NHS calls a medical emergency, so it needs a 999 call.

What else do the sources say?

Some people feel better soon after starting levothyroxine, while others do not notice a change in symptoms for several months. An overactive thyroid that is not treated or is poorly controlled can lead to eye problems and pregnancy complications as well as thyroid storm. A high TSH with a low thyroid hormone level in the blood could mean an underactive thyroid, while a low TSH with high thyroid hormone levels usually means an overactive thyroid. Radioactive iodine is not suitable for everyone, for example people who are pregnant or breastfeeding, and surgery is usually kept for cases such as a large goitre or when other treatments cannot be used.