Overactive thyroid (hyperthyroidism)

What is an overactive thyroid?

Your thyroid gland is a butterfly-shaped organ in the front of your neck.  It produces hormones that help control your metabolism — processes that determine how your body processes energy.  Your metabolism affects how almost every single organ in your body works. These hormones are:

• thyroxine (T4)
• triiodothyronine (T3)

If your thyroid gland produces too many of these hormones, it's called an overactive thyroid (hyperthyroidism). 

You can check your thyroid levels at home by using a thyroid blood test.

Symptoms of an overactive thyroid

An overactive thyroid often causes your body’s functions to “speed up”. This might cause: 

• weight loss despite a normal or increased appetite
• excess sweating and heat intolerance
• muscle weakness
• shaking and tremors
• heart palpitations
• irregular periods
• increased irritability or anxiety
• diarrhoea

Causes of an overactive thyroid

There are a few things that can cause an overactive thyroid. 

Graves' disease 

One of the most common causes of an overactive thyroid is Graves' disease. It’s an autoimmune condition caused by the abnormal production of auto-antibodies, which means your immune system mistakenly targets your thyroid.  This results in the overproduction of thyroid hormones.  

Pregnancy

During pregnancy, your placenta produces human chorionic gonadotrophin (hCG). This hormone is very similar to thyroid-stimulating hormone (TSH), so hCG can weakly bind to your thyroid receptors. Your hCG levels are highest:

• during early pregnancy
• if you’re pregnant with more than one foetus
• If you’re experiencing a molar pregnancy — a condition where a lump of abnormal cells grows in the womb instead of a foetus.

When hCG levels are very high, it can sometimes stimulate your thyroid gland enough to produce thyroid hormones.

Thyroid nodules (Plummer disease)

Nodules are non-cancerous growths on your thyroid gland. This can cause your thyroid gland to secrete excess hormones. When one or more of these nodules are present on an enlarged thyroid gland, it's called a multinodular goitre.

Thyroiditis

Thyroiditis is when your thyroid gland becomes inflamed or swollen — which can result in either an overactive or underactive thyroid. Causes of thyroiditis that lead to an overactive thyroid include:

• subacute (De Quervain’s) thyroiditis — when a viral infection causes your thyroid gland to become inflamed
• silent (transient or painless) thyroiditis — when your immune system targets your thyroid gland
• postpartum thyroiditis — when your immune system targets your thyroid gland after giving birth 
• radiation-induced thyroiditis — sometimes caused by radiotherapy or radioactive iodine treatment
• acute (infectious) thyroiditis — usually caused by a bacterial infection  

Some medicines

Thyroid hormones are made up of iodine molecules.  Some medicines containing iodine can cause your thyroid to become overactive —this is known as drug-induced thyroiditis. Your thyroid function will improve soon after stopping the medication, but your thyroid levels might continue to be high for a few months. Other medicines containing lithium and amiodarone can also have an effect on your thyroid. 

Thyroid cancer

In rare cases, a cancerous growth on your thyroid gland might cause your thyroid gland to become overactive. 

Diagnosing an overactive thyroid with a blood test 

A thyroid blood test can measure the following:

• triiodothyronine (T3) — one of your main thyroid hormones
• thyroxine (T4) — another main thyroid hormone
• thyroid-stimulating hormone (TSH) — a hormone secreted by the pituitary gland at the base of the brain that controls the production of T3 and T4
• thyroid antibodies — in the case of Graves' disease, your immune system might target your thyroid, which will cause thyroid antibodies in the blood. 

If you have higher T3 and T4 levels but lower TSH levels, it could be a sign of an overactive thyroid. Your GP might refer you to a specialist to find the cause. 

Treating an overactive thyroid

An overactive thyroid can be treated. Main treatment methods include:

• medicines called thionamides, like carbimazole or propylthiouracil. These help your thyroid gland stop overproducing hormones
• radioactive iodine treatment — to destroy cells in your thyroid gland so that fewer hormones are produced 
• surgery — to remove parts of your thyroid gland if medicine or radioactive iodine therapy doesn’t work.

Medicines known as beta-blockers might also be given to alleviate some symptoms, like tremors and palpitations until the treatment takes effect.