Thyrotoxicosis

Thyrotoxicosis is a medical condition caused by an excessive amount of thyroid hormones in the bloodstream.

Alternative names for thyrotoxicosis

The term thyrotoxicosis is sometimes interchanged with hyperthyroidism though their definitions are slightly different.

What is thyrotoxicosis?

Bulging left eye of a 58-year-old man with thyrotoxicosis.

Bulging left eye of a 58-year-old man with thyrotoxicosis.

Thyrotoxicosis is the name given to the clinical effects experienced due to an excess of thyroid hormones in the bloodstream. Hormones produced by the thyroid gland control how fast or slow the body works (basal metabolic rate). Too much thyroid hormone (thyroxine and triiodothyronine) can therefore cause an increase in how fast the body functions. This results in the signs and symptoms of thyrotoxicosis.

What causes thyrotoxicosis?

The main cause of thyrotoxicosis is hyperthyroidism, which is an overactivity of the thyroid gland resulting in it producing excess levels of thyroid hormones. If the hyperthyroidism is due to an autoimmune cause, it is called Graves' disease.

Other rarer causes of thyrotoxicosis include excessive intake of thyroid hormone in those patients treated for hypothyroidism (an underactive thyroid gland) and sometimes, inflammation of the thyroid gland (thyroiditis), which can lead to damage to the gland. This causes release of large quantities of thyroid hormone into the bloodstream. Other conditions that may cause excessive thyroid hormone production include toxic goitre/'>multinodular goitre and toxic thyroid adenoma. Certain medications, for example, amiodarone and lithium, can also cause excessive thyroid hormone production.

What are the signs and symptoms of thyrotoxicosis?

The symptoms of thyrotoxicosis are a result of the high levels of thyroid hormones in the blood, increasing the rate at which the body is working. These can include diarrhoea, weight loss (although around 10% develop weight gain), shaking or tremor (notably in the hands), sweating, increased heart rate that can be felt by the patient (palpitations), overactivity, agitation, anxiety, changes in emotions and feeling hot even though the room may be cold to everyone else. Patients may also experience an increased heart rate, thinning of the hair, swellings or nodules in the thyroid gland (goitre) or other signs of hyperthyroidism. Certain hyperthyroid signs are seen in patients with the autoimmune cause and include prominent eyeballs (thyroid eye disease), skin changes on the legs and swellings on the fingers.

In women, it can also affect menstruation and cause irregular periods and, if severe, even infertility. Other rarer symptoms include hair loss (alopecia) and itching.

How common is thyrotoxicosis?

Thyroid disease tends to be commoner in women than men (up to 10 times more). Thyrotoxicosis can occur at any age and there is a 1 in 100 chance of a woman developing it in her lifetime and a 1 in 1,000 risk for men. Around 2% of the female population has the condition. The number of new cases diagnosed each year is 3 per 1,000 women. 

Is thyrotoxicosis inherited?

The main cause of thyrotoxicosis is hyperthyroidism, which 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 the children will develop the condition. The other causes of thyrotoxicosis are not known to be inherited.

How is thyrotoxicosis diagnosed?

The diagnosis is based on the combination of symptoms, as described above, together with abnormalities in blood tests used to examine thyroid hormone levels. To diagnose thyrotoxicosis, tests should show raised thyroid hormone levels in the blood (raised thyroxine and/or triiodothyronine) and low, undetectable thyroid stimulating hormone (a hormone released by the pituitary gland that stimulates the thyroid gland to release its hormones). Thyroid stimulating hormone becomes suppressed in this way due to the excessive thyroxine and triiodothyronine already circulating in the body. This is sensed by the pituitary gland, which inhibits it from producing any more thyroid stimulating hormone.

If the patient is suspected of having autoimmune hyperthyroidism (Graves’ disease), an additional blood test to check for auto-antibodies may be required.

How is thyrotoxicosis treated?

Exact treatment depends on the cause of the condition. There are three main methods of treatment for thyrotoxicosis:

  1. Medication – drugs called beta-blockers (e.g. propranolol), can be used to reduce the symptoms of thyrotoxicosis such as the heart rate, anxiety or sweating. However, to treat the raised hormone levels, different medication called carbimazole or another called propylthiouracil is used. These act on enzymes in the thyroid gland to prevent production of thyroid hormones.

    Patients can be treated using two different medication regimes. They can start by taking carbimazole every day with thyroid function tests regularly (every one to two months) and have the medication dose changed according to the blood test results. Alternatively, they can have treatment with both carbimazole and manufactured thyroid hormone, started at the same time. This second method causes less chance of overtreatment, which may cause the patient to become hypothyroid. It also leads to more stable thyroid blood tests.
     
  2. Radioactive iodine – this involves the patient taking either a capsule or liquid containing radioactive iodine. This can be carried out in the outpatient department. Iodine is used mostly by the thyroid gland so that all the radioiodine is concentrated in the gland and causes a gradual destruction of it.
     
  3. Surgery – this involves removal of all or part of the thyroid gland. This is normally only performed once thyroid hormone levels have been reduced with medication.

Are there any side-effects to the treatment?

If medication is used to treat the thyrotoxicosis, it is normal for it to take a little time to find the right dose of medication to suit the individual. Therefore, the doctor will request that the patient has regular blood tests, particularly at the start of treatment, so that the levels of thyroid hormones in the blood can be measured and the medication regime altered if levels are too high or too low.

As with most treatments, there are side-effects associated with each of the treatment options: 

  • Although carbimazole is relatively safe to take, on rare occasions it can cause reduced white blood cell production by the body. It is important to alert the doctor if the patient has a sore throat, a fever or develops mouth ulcers when on the medication. This side-effect is very rare with only 0.03% of patients developing it.
     
  • Radioactive iodine can cause a sore throat for a few days in a small minority of patients. It is also important to stay away from children and pregnant women for three weeks due to the risk of exposing them to radiation. It is also recommended that female patients avoid pregnancy for about six months after the treatment has finished. The exact lengths of time should be discussed with the doctor. There is also a 50–80% risk of developing hypothyroidism due to destruction of the thyroid gland.
     
  • There are general risks with surgery and anaesthesia, which should be reviewed by the surgeon or anaesthetist. Further side-effects, which occur in up to 2% of cases, include damage to the recurrent laryngeal nerve, which runs close to the thyroid gland in the neck. This can affect or alter a patient’s voice if it is damaged (causing a hoarse voice). The parathyroid glands are located behind, and attached to, the thyroid gland and can rarely be completely removed by the surgery causing low blood calcium hypoparathyroidism. However, in some cases, one of the parathyroid glands can be preserved in the body.

What are the longer-term implications of thyrotoxicosis?

Long-term implications of treated thyrotoxicosis depend on which of the treatment methods are used, and generally tend to be the need for lifelong medication taken daily. Patients on medication should have blood tests taken at regular intervals to check the effectiveness of the treatment. 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. Long-standing thyrotoxicosis can also affect the patient’s bones and cause osteoporosis, which results in weak bones that are more likely to fracture.


Last reviewed: Jan 2015