Underactive thyroid (hypothyroidism)

If the thyroid doesn't produce enough hormones, your metabolism (a set of chemical reactions in the body) slows down. That can affect various body functions and cause a number of different symptoms. For example, a reduction in physical fitness. Symptoms of this might include:

• Weakness and tiredness
• Low pulse
• Shortness of breath

Mental health or cognitive problems are also possible. For instance:

• Problems with concentration or memory
• Lethargy
• Depression or other psychological problems

It can lead to physical changes too, such as:

• Dry skin, sometimes puffy and thickened – especially in the face
• Dry hair, hair loss
• Slight to moderate weight gain
• Enlarged thyroid (goiter)
• Low, hoarse voice
• Enlarged tongue
• Constipation
• Heavy periods or other menstrual problems, reduced fertility
• Erectile dysfunction
• Poor hearing

Other symptoms may affect your entire body, like:

• Sensitivity to cold, chills
• Muscle or joint pain
• Tingling or numbness
• Slow reflexes

There are various reasons why the thyroid gland might not produce enough hormones:

• Inflammation of the thyroid gland: The organ may become inflamed due to an autoimmune reaction. This is known as Hashimoto's disease (or Hashimoto’s thyroiditis). It is the most common cause of an underactive thyroid in Germany and other countries. The inflammation may be caused by an infection instead. Sometimes no clear cause is found.
• Thyroidectomy or radiation therapy: Sometimes treatment for an overactive thyroid gland or thyroid cancer involves using radioactive iodine (radioiodine) on the thyroid gland, or surgically removing part or all of the gland (thyroidectomy). As a result, not enough thyroid hormones are made, or none are made at all.
• Severe iodine deficiency: The body needs the trace element iodine in order to produce thyroid hormones. Eating a balanced diet and using iodized salt can help make sure you get enough iodine. Iodine-rich foods include seafood, milk, dairy products and eggs.
• Medications: An underactive thyroid can also be caused by medication, for example as a side effect of an antidepressant that contains lithium. Other medications may be used especially to reduce the production of thyroid hormones in people who have an overactive thyroid. If the dose is too high, the thyroid might become underactive.
• Congenital (“at birth”) hypothyroidism: People are only rarely born with an underactive thyroid because of their genes. It can also develop if a pregnant woman doesn't get enough iodine or if her immune system makes antibodies against the thyroid tissue.

In all of these forms, the thyroid gland itself no longer works properly. Doctors call this "primary hypothyroidism." Only very rarely is hypothyroidism caused by a problem with the pituitary gland or hypothalamus in the brain. These organs regulate the production of hormones in the thyroid gland. If they aren't working properly, the thyroid gland won't produce enough hormones even if it's healthy. This is called central hypothyroidism.

About 5 out of 100 people have an underactive thyroid in countries like Germany. It is especially common in women and in older age. About 1 out of 3,400 babies are born with an underactive thyroid.

How an underactive thyroid develops over time will depend on what's causing it. It often develops slowly because it is very commonly caused by an inflammation of the thyroid gland. This usually leads to the gradual loss of thyroid tissue, which can go unnoticed for a long time: The remaining tissue makes up for this by producing more hormones. But symptoms arise when that is no longer possible. If an underactive thyroid isn't treated, various problems can develop.

Sometimes the thyroid gets better on its own, though – for instance, if the inflammation goes away.

Complications can arise over the long term if an underactive thyroid isn't treated. The effects can be serious, especially in children. Some are permanent – such as dwarfism (short height), partial or full loss of hearing, and problems with mental and cognitive development. This is very rare in Germany and other industrialized countries though, because it's usually detected in newborn check-ups ( screening) shortly after birth and then treated in time.

If an underactive thyroid is left untreated in adults, complications such as cardiovascular (heart and blood vessel) disease, confusion and drowsiness are possible. In very rare cases it can lead to a coma.

Your doctor will ask about your symptoms and examine the thyroid by feeling your neck. Because many of the symptoms of an underactive thyroid could have other causes, blood tests are done to get a clear diagnosis.

The tests measure the levels of various hormones in your blood. These include the thyroid hormones themselves. But another hormone called TSH (thyroid-stimulating hormone) is important too. It is produced in the pituitary gland, not in the thyroid. As its name suggests, TSH stimulates the thyroid gland to produce thyroid hormones.

A higher level of TSH in your blood may be a sign of an underactive thyroid. This is because as soon as the thyroid gland gets weaker, the pituitary gland responds: It increases the amount of TSH it makes in order to stimulate the thyroid to make more thyroid hormones. But this is not enough if the thyroid is very underactive: The thyroid still can't release enough hormones into the bloodstream. This can also be seen in a blood test.

Newborns are routinely screened for high TSH levels in order to detect an underactive thyroid early on so that it can be treated sooner. If the results are abnormal, doctors also measure the level of thyroid hormones.

Screening is not recommended in adults because abnormal TSH levels don't automatically mean that the thyroid isn't producing enough hormones and causing problems. What's more, TSH levels that are just a little high very often go down on their own, but it can sometimes take a while. So abnormal levels could lead to unnecessary further testing, treatments and worry.

If your TSH is high but your thyroid gland is still making enough hormones and you don't have any symptoms, it's known as subclinical (latent) hypothyroidism.

Each year in Germany, 2 to 5 out of 100 people who have subclinical hypothyroidism develop an underactive thyroid with symptoms. It's not clear whether this can be prevented with early treatment, though.

You can't prevent a thyroid gland from becoming underactive – with one exception: An underactive thyroid caused by iodine deficiency can be prevented by getting enough iodine. Experts recommend that you make sure that you get enough iodine in your diet. You can do this by having milk, dairy products or eggs on a daily basis and eating saltwater fish regularly. The use of iodized table salt is also recommended.

Some dietary supplements have iodine in them too. Taking them on a daily basis can cause problems if the supplements contain more than 100 µg (micrograms) of iodine. If you take too much iodine over the long term, it increases your risk of developing an overactive thyroid. It is almost impossible to get too much iodine from food and drink, though. One exception is dried algae, especially seaweed and kelp.

If the levels of thyroxine (a thyroid hormone made in the body) are too low, a medication that is transformed into the hormone inside the body is used to replace it. This medication is called levothyroxine or L-thyroxine. It keeps hormone levels within the normal range, and the symptoms typically go away completely. It takes about two to three months for the body's hormone levels to return to normal. Depending on the cause, treatment for an underactive thyroid may be temporary or lifelong.

Taken at the right dose, side effects are very rare. The dose is determined based on the person’s body weight. If the dose is too high, it can cause agitation. So the dose is checked regularly during the first few weeks of treatment and changed as needed. After that, the hormone levels usually only have to be checked once a year.

Doctors disagree about whether subclinical hypothyroidism should be treated.

When people are ill or need medical advice, they usually go to see their family doctor first. Information about health care in Germany can help you to navigate the German health care system and find a suitable doctor. You can use this list of questions to prepare for your appointment.