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Hyperthyroidism

Print Print | Email  Email article to a friend | Last updated: January 07, 2015

Hyperthyroidism is a common condition where the thyroid gland becomes overactive and produces too much thyroid hormone.

Alternative names for hyperthyroidism

Overactive thyroid. The term hyperthyroidism is often interchanged with thyrotoxicosis although their definitions differ slightly. See the article on thyrotoxicosis for more information.

What is hyperthyroidism?

The thyroid gland produces hormones which regulate the body’s metabolic rate as well as heart and digestive function, muscle control, brain development and bone maintenance. Hyperthyroidism occurs when the thyroid gland becomes too active and produces too much thyroid hormone. This leads to symptoms such as tiredness, increased sweating and weight loss.

What causes hyperthyroidism?

There are a number of different causes of hyperthyroidism including:

  1. Autoimmune thyroid disease or Graves' disease – the most common cause of hyperthyroidism (80% of all cases). This affects women more commonly than men and is due to the production of special proteins (called antibodies) that attack the thyroid gland. The antibodies fool the thyroid gland into thinking that more thyroid hormone production is needed, which results in hyperthyroidism. 
     
  2. Toxic thyroid nodule or nodules – the second most common cause of hyperthyroidism (15% of cases). One or several thyroid nodule(s) (lumps) work independently from the rest of the thyroid gland and produce too much thyroid hormone.
     
  3. Thyroid inflammation or thyroiditis – a rare cause of hyperthyroidism (1-2% of cases) usually due to a viral illness, which causes inflammation and tissue destruction of the thyroid gland, leading to release of stored thyroid hormones. The neck may be tender to touch over the thyroid, but this condition usually settles without specific treatment. The thyroid gland usually regains normal function; however, in a minority of patients the thyroid can become underactive (hypothyroidism).
     
  4. Drugs – drugs being used to treat other conditions can cause hyperthyroidism.  The most common is amiodarone, a drug used to control an irregular heartbeat.
     
  5. There are some other extremely rare causes of hyperthyroidism such as benign tumours of the pituitary gland that produce an excess of thyroid stimulating hormone.

What are the signs and symptoms of hyperthyroidism? 

In more than 75% of cases symptoms of hyperthyroidism include: nervousness, irritability, the inability to relax, feeling warm, heat intolerance, increased sweating, palpitations, fatigue, increased frequency of bowel movements, increased appetite and weight loss.

In around 20-30% of cases difficulties in sleeping (insomnia) and irregular periods can occur.

Graves’ disease can be linked to symptoms in parts of the body other than the thyroid.  These include:

  • Thyroid eye disease – redness and inflammation of the eyes. Sometimes the eyeballs are pushed forward resulting in ‘eye bulging’. It is essential that smokers who develop this complication give up smoking immediately.
     
  • Pretibial myxedema – a skin condition that usually affects the shins.

In about 25% of cases of Graves' disease, there are signs of thyroid eye disease and more rarely (about 5%) pretibial myxoedema. Typically there is also an enlargement of the thyroid gland which is known as a goitre.

How common is hyperthyroidism?

Hyperthyroidism is more common in women than men, and affects two in 100 women and two in 1,000 men.

Is hyperthyroidism inherited?

Hyperthyroidism can run in families; however, there is no single gene that is responsible for this condition. Individuals may inherit a greater likelihood of having Graves’ disease, the commonest cause of hyperthyroidism, but if one of the parents is affected, it does not necessarily mean that children will develop the condition.  

How is hyperthyroidism diagnosed?

A full clinical and family history should be taken and thorough examination carried out by a doctor. Simple blood tests called thyroid function tests will then be carried out to confirm the diagnosis. These tests measure the amount of thyroid hormones (triiodothyronine and thyroxine) and thyroid stimulating hormone in the bloodstream. In hyperthyroidism, the levels of triiodothyronine and/or thyroxine are usually raised, with undetectable levels of thyroid stimulating hormone. When the condition is in its early or mild stage, triiodothyronine and thyroxine can be in the normal range with suppressed thyroid stimulating hormone; this is known as subclinical hyperthyroidism.

Usually, the thyroid function tests together with the clinical examination are enough to diagnose the cause of hyperthyroidism. However, in some cases antibody testing may be needed to confirm the diagnosis, and occasionally, thyroid iodine uptake scans are requested to identify the cause (usually when a patient has the combination of a thyroid nodule and hyperthyroidism, or if thyroiditis is suspected). This is a test to measure how much iodine is taken up by the thyroid gland and gives an indication of thyroid function.

How is hyperthyroidism treated?

There are three treatment options for hyperthyroidism:  

  1. Medical treatment – drugs called carbimazole or propylthiouracil are given in tablet form. These are almost always the first choice of treatment to get the thyroid gland under control. There are two main ways to give these drugs:
    1. block and replace – a high dose of the drug stops the thyroid gland working altogether and this is combined with replacement doses of thyroid hormone. This treatment is not safe in pregnancy.
       
    2. titration regime – enough of the drug is given to partly suppress the thyroid gland and keep the body’s natural thyroid hormone levels within a normal range.
       
  2. Radioactive iodine treatment – this is usually given as a capsule which can be done as an outpatient with no hospital admission necessary. The contents of the capsule are absorbed and concentrated in the thyroid gland causing a gradual destruction of the gland. The iodine does not affect any other part of the body and is a relatively safe treatment. Around 90% of patients respond to a single dose but a second and very rarely third dose may be required.
     
  3. Surgical removal of all or part of the thyroid gland (thyroidectomy); this would be carried out as a hospital in-patient.

Are there any side-effects to the treatment?

  1. Tablets – very rarely these can suppress the production of white blood cells, making the individual more open to infections. If patients experience a sore throat, mouth ulcers or high temperature whilst on this treatment they should seek medical attention immediately (a simple blood test is enough to diagnose this complication). The tablets should be stopped until the result of the blood test is known. Although very rare, this complication can be fatal if the diagnosis is delayed. Individuals who are confirmed as having white cell suppression are admitted to hospital and intravenous antibiotics are given until the white cell count recovers.
     
  2. Radioactive iodine – this can cause permanent hypothyroidism (an underactive thyroid) requiring lifelong thyroid hormone replacement. Special precautions should be taken after radioiodine treatment, such as limiting contact with young children and pregnant women for three weeks to avoid exposing them to radiation. This form of treatment should never be used for pregnant women. There is a risk that radioiodine treatment may worsen thyroid eye disease, so it is not given to patients with moderate to severe eye disease. 
  3.  
  4. Surgery – there are general risks of surgery and anaesthesia which should be reviewed by the surgeon or anaesthetist. Further rare side-effects include damage to the recurrent laryngeal nerve which runs close to the thyroid gland in the neck and can affect or alter a patient’s voice if damaged (causing a hoarse voice). The parathyroid glands are attached to the thyroid and can be temporarily or permanently injured during the surgery causing hypoparathyroidism. This would require the patient to take calcium and vitamin D supplements for life.  However, in some cases one of the parathyroid glands can be preserved in the body and normal parathyroid function is restored. 

What are the longer-term implications of hyperthyroidism?

The long-term implications of hyperthyroidism depend on the treatment option used.  Patients taking carbimazole tablets will likely need to take them daily for life. Blood tests should be carried out regularly to monitor thyroid hormone levels and to adjust the dose of carbimazole accordingly. The majority of patients are able to find a medication regime that works for them and go on to live full, active lives.

If untreated, besides feeling poorly and unwell, the patient is also at risk of heart dysfunction or failure due to the increased heart rate and raised metabolic state. This irregular heart rate can result in strokes and dizziness. An overactive thyroid can also affect the patient’s bones and cause osteoporosis which results in weak bones which are more likely to fracture.   

 

Reviewed: January 2015

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