Underactive thyroid

An underactive thyroid (hypothyroidism) occurs when the thyroid gland does not produce enough hormones. This slows down the metabolism. People affected by it often feel lethargic, have a low heart rate and constipation. A blood test is a reliable way to diagnose hypothyroidism.

At a glance

  • If the thyroid gland does not produce enough hormones this is referred to as an underactive thyroid (hypothyroidism).
  • An underactive thyroid slows down the metabolism.
  • Symptoms include, for example, fatigue, listlessness or constipation.
  • The most common cause of hypothyroidism is Hashimoto’s thyroiditis. This is an autoimmune disease of the thyroid gland where the body’s immune system identifies the thyroid cells as invaders and sends antibodies to attack them.
  • In adults hypothyroidism often develops insidiously as it is mostly caused by Hashimoto’s thyroiditis.
  • In older people in particular, hypothyroidism may be mistaken for age-related changes such as incipient dementia.

Note: The information in this article cannot and should not replace a medical consultation and must not be used for self-diagnosis or treatment.

Hypothyroidism: woman clutching her throat.

What is an underactive thyroid?

An underactive thyroid (hypothyroidism) occurs when the thyroid gland does not produce enough hormones. The thyroid gland is a vital organ. It produces hormones that regulate metabolic processes and keep them in balance. The thyroid hormones control a large number of processes in the human body.

An underactive thyroid slows down the metabolism. This can lead to a number of problems that are relatively non-specific. Fatigue, listlessness or constipation are some of the symptoms.

Most cases of an underactive thyroid are caused by a chronically inflamed thyroid gland. An underactive thyroid is easily treated by taking one thyroid hormone tablet a day to restore hormone levels. The problems normally clear up as a result of this replacement hormone therapy.

What are the common forms of thyroid disease?

This video explains what the most common forms of hypothyroidism and hyperthyroidism are, what causes them and how they are treated.

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What symptoms indicate an underactive thyroid?

Exhaustion, debility and fatigue, swelling of skin, mild to moderate weight gain, enlarged thyroid, depression, concentration and memory problems.

If the thyroid gland does not produce enough hormones this slows down the metabolism, which may affect many parts of the body and cause very specific problems. Potential symptoms include:

  • exhaustion, weakness and fatigue
  • low heart rate
  • sensitivity to cold and decreased sweat production
  • shortage of breath (dyspnea) during exertion
  • mild to moderate weight gain
  • difficulty concentrating or memory problems, slow thoughts or speech
  • constipation
  • dry skin and hair, hair loss
  • yellowing of skin
  • low-pitched, hoarse voice
  • enlarged tongue
  • puffy, swollen skin
  • enlarged thyroid (also referred to as struma or goiter)
  • hearing impairment
  • muscle pain or joint pain
  • slower reflexes
  • heavy or irregular periods, fertility problems
  • erectile dysfunctions
  • lethargy
  • depression or other mental abnormalities

Many of these symptoms may have other causes too. In older people in particular, an underactive thyroid may be mistaken for age-related changes. This may happen, for example, in people with incipient dementia. So blood tests are essential to confirm a suspected diagnosis of an underactive thyroid.

What causes an underactive thyroid?

An underactive thyroid may have a range of causes.

A hormone deficiency can be caused by a thyroid gland disorder or other factors. Possible causes include:

  • Hashimoto’s thyroiditis: This is an autoimmune disease of the thyroid gland. It is the commonest cause of an underactive thyroid. The body’s immune system mistakes the thyroid cells for invaders. Its antibodies therefore attack its own thyroid gland, causing inflammation that is restricted to the thyroid gland.
  • Removal of thyroid gland or radiotherapy: Treatment of some conditions such as an overactive thyroid and thyroid cancer involves exposing the thyroid gland to radiation, or surgery to remove all or part of it. An enlarged thyroid gland (struma) is also treated by surgically removing parts of the thyroid gland. This means that not enough thyroid hormones are produced, or sometimes none at all. Radiotherapy can lead to an underactive thyroid developing very slowly and only occurring several years later.
  • Severe iodine deficiency: Iodine is a trace element. The body needs it to produce the thyroid hormones. Since the body cannot produce iodine itself it must get it from food. If the body does not get enough iodine from food over a long period it may develop an underactive thyroid. However this is very rarely the case in industrialized countries.
  • Medication: Certain types of medication may inhibit the production of thyroid hormones. These include various cancer drugs as well as the active substance lithium. Lithium is used to treat certain mental illnesses. But some drugs that are taken for an overactive thyroid can also inhibit production of thyroid hormones. If the dosage is too high this can cause an underactive thyroid.
  • Central hypothyroidism: An underactive thyroid can also be caused by abnormal function of the pituitary gland or the hypothalamus. But this only applies to less than one per cent of those affected. The pituitary gland and hypothalamus produce hormones that control hormone production in the thyroid gland. They are part of the central nervous system.

How does an underactive thyroid progress?

How an underactive thyroid progresses varies depending on what causes it. Most adults have Hashimoto’s thyroiditis. This normally causes a gradual decomposition of the thyroid tissue. In these cases an underactive thyroid often develops insidiously. The remaining thyroid tissue can offset the loss for some time. It produces more hormones to meet the body’s needs. But as the condition progresses more and more tissue is lost, until the gland does not produce enough hormones. Symptoms then appear.

Tablets can be taken to easily replace the missing thyroid hormones. This restores normal metabolism and the symptoms disappear. If left untreated, an underactive thyroid can cause a number of complications. In (rare) extreme cases it may cause a life-threatening coma.

Tablets can be taken to replace the missing thyroid hormones.

How can an underactive thyroid be prevented?

Iodine deficiency is now only very rarely the cause of an underactive thyroid in Germany. An underactive thyroid caused by iodine deficiency can be prevented by ensuring an adequate intake of iodine. Iodized salt is one of the best sources of iodine. There are also certain food supplements that contain iodine.

The recommended daily intake of iodine varies according to age. The recommended daily iodine intake for adults is 200 micrograms (µg) but should not exceed 500 µg. Pregnant and breastfeeding women need a bit more and should consult their doctor for specific recommendations. Children need less than 200µg of iodine per day.

The Robert Koch Institute conducted a study into iodine levels of children and adolescents up to 18 in 2007. The study found that iodine levels in around 85% of those under 18 were adequate or more than adequate. There is still no extensive data on this for adults.

Important: Excessive iodine levels are also a problem in the long term. They increase the risk of an overactive thyroid. However it is virtually impossible to overdose from food. The only exception to this is dried algae and seaweed in particular. They contain so much iodine that people could exceed the maximum daily recommended dose of 500 µg. Food supplements can also be a problem if the daily dose contains more than 100 µg of iodine.

How can an underactive thyroid be detected early on?

A normally functioning thyroid gland is particularly important for babies and children. An underactive thyroid gland can have a hugely adverse effect on physical and mental development. As a result, thyroid hormone levels are routinely measured for all newborns to ensure any treatment needed is initiated as early as possible. The likelihood of being born with an underactive thyroid is very low: it affects just one out of every 4,000 newborns.

There is no scientific evidence of the benefits of measuring adult hormone levels for screening purposes. However there are a number of risks associated with the method. A relatively large number of people have slightly elevated TSH levels. It is estimated that this occurs in 5 to 10% of the population. TSH stands for “thyroid stimulating hormone”, a hormone that is produced in the pituitary gland and released to stimulate production by the thyroid gland. The TSH level can be used to determine whether the thyroid gland is working properly. If the TSH level is elevated but enough thyroid hormones are still being produced, this is referred to as “latent hypothyroidism”.

Important: TSH levels are not conclusive on their own. A routine test could entail unnecessary treatment and worry. That’s because elevated TSH levels do not necessarily mean a diagnosis of symptomatic hypothyroidism.

On average, just 3 to 4 people out of a hundred with elevated TSH levels go on to develop symptoms that require treatment. If the TSH levels are very high, or the blood contains certain antibodies, the risk is higher. Women are at a higher risk than men.

There are also other arguments against routinely measuring TSH levels without a specific reason. As yet there is no generally accepted limit above which TSH levels should be classed as too high. Hormone levels also undergo natural fluctuation. So one abnormal reading on its own is not evidence of the condition. Slightly elevated TSH levels are also reversed relatively often, sometimes after long periods. 

TSH levels increase a little with advancing age. So there is some discussion as to whether it would be a good idea to use different limits for older and younger people. TSH levels are also often a little higher in children and adolescents as their bodies are still growing.

How is an underactive thyroid treated?

At the start of the examination the doctor asks the patient about their symptoms. The doctor will then palpate the thyroid gland. If there is evidence of an underactive thyroid this is followed by a blood test. The TSH levels are measured first. Slightly elevated TSH levels may be the first sign of an incipient underactive thyroid. However there is no scientific consensus over the precise point at which TSH levels should be classed as elevated. For many people, TSH levels are abnormal if they exceed 4 to 5 milli-units per liter (mU/L), for others if they exceed 2.5 mU/L. Occasionally the TSH level may only be temporarily elevated and will return to normal by itself. So it is best to take a second measurement.

An underactive thyroid is present if:

  • typical symptoms occur.
  • the thyroxine level (fT4) is too low.

The normal range for thyroxine levels may differ slightly between laboratories due to the different methods used for measurement. The normal range is therefore always specified by the laboratory performing the test.

How is an underactive thyroid treated?

Treatment aims to compensate for the hormone deficit. The missing thyroid hormone (thyroxine) is replaced by a drug that is identical to the hormone produced by the body. The active ingredient is also referred to as levothyroxine or L-thyroxine. The drug restores hormone levels to normal so that symptoms usually clear up entirely. Depending on the cause, treatment of an underactive thyroid is either temporary or lifelong.

There are virtually no side effects provided the right dose is given. The dosage is based on body weight. It is adjusted after 2 to 3 months as required. That’s how long it takes for the hormone levels to stabilize.

In latent hypothyroidism it is only the TSH levels that are elevated; hormone levels remain within the normal range. There is still no expert consensus on the point at which treatment should start. Immediate treatment is not usually required.

For more detailed information, such as how to treat an underactive thyroid and when, visit gesundheitsinformation.de.

In cooperation with the Institute for Quality and Efficiency in Health Care (Institut für Qualität und Wirtschaftlichkeit im Gesundheitswesen) (IQWiG). As at:

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