Are there benefits to treating subclinical hypothyroidism?

In subclinical (latent) hypothyroidism, the thyroid gland is still producing enough thyroid hormones. But abnormal amounts of other substances in the blood suggest that the thyroid hormone levels are starting to go down. It is still unclear whether there are benefits to treating subclinical hypothyroidism.

If you already have noticeable (overt) hypothyroidism, the thyroid is no longer producing enough thyroid hormones. Thyroxine is the main thyroid hormone. It helps regulate many of the body’s functions and balances your metabolism (chemical reactions in the body). Too little thyroxine can cause a number of different health problems. The symptoms range from skin changes to physical weakness, difficulty concentrating and depression. Hypothyroidism can be easily treated by taking a hormone tablet once a day. These tablets replace the thyroxine that is not being made by the body. The symptoms usually disappear completely as a result.

Subclinical hypothyroidism doesn't cause any noticeable symptoms. The levels of TSH (thyroid-stimulating hormone) are too high, but the thyroid gland is still producing enough hormones. TSH is made in the pituitary gland in the brain. It stimulates the thyroid gland to make thyroid hormones. TSH levels that are just a little too high may be the first sign of early stages of hypothyroidism. The pituitary gland detects lower levels of thyroid hormones in the blood and then increases TSH production to activate the thyroid gland to make more hormones.

It is estimated that about 5 out of 100 people have subclinical hypothyroidism. But slightly elevated TSH levels don't pose any health risk themselves. TSH levels are often only temporarily higher, for example after intense physical activity. So there is some disagreement about when subclinical hypothyroidism should be treated.

How is subclinical hypothyroidism diagnosed?

High TSH levels might be detected during a routine blood test, for instance. TSH levels are also measured if you have another medical condition and certain treatments or examinations are planned, for example using contrast media (dye-like substances) containing iodine. These treatments or examinations could lead to complications if you have thyroid problems, so doctors check whether your thyroid is healthy beforehand. Subclinical hypothyroidism is diagnosed if the TSH level is high but the thyroid hormone thyroxine is still within the normal range. The TSH test is usually done twice just to be sure.

There is some disagreement among experts regarding which TSH levels should be considered too high. Some say that TSH levels of over 2.5 milliunits per liter (mU/L) are abnormal, while others only consider TSH levels above 4 to 5 mU/L to be too high.

Children, teenagers and older people have somewhat higher TSH levels than middle-aged people do. Because of this, thyroid specialists have been debating whether a higher threshold should be used for those age groups. Being severely overweight and using certain medications can also increase your TSH levels. TSH levels can go up and down a bit during pregnancy.

How does subclinical hypothyroidism develop over time?

The way that subclinical hypothyroidism develops over time depends on a number of different factors – including the TSH levels: Slightly high TSH levels (between 5 and 10 mU/L) often return to normal on their own. But people who have very high levels (over 15 mU/L) often develop overt hypothyroidism with noticeable symptoms after several months or years.

One study followed people with high levels of TSH over a period of about two to three years. The people didn't have any symptoms or known thyroid problems at the start of the study. They were divided into three groups based on how high their TSH levels were. The study showed the following:

Slightly high TSH levels (between 5 and 10 mU/L): Each year, 2% of the people in this group developed hypothyroidism with symptoms.
Moderately high TSH levels (between 10 and 15 mU/L): Each year, 20% of the people in this group developed hypothyroidism with symptoms.
Very high TSH levels (over 15 mU/L): Each year, 73% of the people in this group developed hypothyroidism with symptoms.

Some people who have high TSH levels never develop symptoms. It is also very common for high TSH levels to return to normal in children and teenagers. So TSH levels that are slightly or only moderately high don't necessarily need to be treated.

Subclinical hypothyroidism is more likely to develop into overt hypothyroidism if the thyroid gland is enlarged and thyroid antibodies are found in the blood. The risk is generally higher in women than in men.

Thyroid antibodies are usually a sign of a condition called Hashimoto’s thyroiditis. This autoimmune disease is the most common cause of hypothyroidism. But having thyroid antibodies in your blood doesn't mean that hypothyroidism will definitely develop.

Does thyroxine treatment have any benefits if your TSH levels are high?

Some doctors will advise you to start treatment immediately if you have subclinical hypothyroidism. This is because there's some evidence to suggest that the risk of cardiovascular disease might slightly increase over the long term if your TSH levels are higher than 10 mU/L. This link hasn't been observed for slightly high TSH levels (lower than 10 mU/L).

Only a few good-quality studies have looked into the potential advantages and disadvantages of thyroxine treatment in subclinical hypothyroidism. The largest and best-quality study involved nearly 800 people over the age of 65. It wasn't large enough to find out whether thyroxine treatment lowers the risk of complications, though. There was also no evidence that the treatment offered other benefits: People who didn't use thyroxine developed symptoms of hypothyroidism just as rarely as people who took thyroxine did.

There are no studies showing any advantages of treating subclinical hypothyroidism in children and teenagers, either. Sometimes young people have higher TSH levels because they are overweight. In that case it doesn't make sense to use thyroxine tablets.

Does thyroxine treatment have side effects?

There's currently a lack of good-quality research on the side effects of treating subclinical hypothyroidism with thyroxine, but it's generally considered to be a very well tolerated drug. Because the body usually makes this hormone on its own, there are no problems as long as the dose is correct. If it's too high, though, side effects can't be ruled out. Possible side effects include heart problems like atrial fibrillation (an irregular heartbeat) and a pounding or racing heartbeat.

Does treatment make sense during pregnancy?

Subclinical hypothyroidism is sometimes treated in pregnant women too. To find out whether they have this condition, they are offered a diagnostic screening test. In Germany this is an “individual health care service” (IGeL Leistung), which you have to pay for yourself.

Some evidence suggests that subclinical hypothyroidism during pregnancy can increase the risk of miscarriage or premature birth. But there is no proof that treatment with thyroxine can lower this risk in women who have high TSH levels or thyroid antibodies. The largest study on thyroxine treatments in pregnancy didn't find any advantages here – for the mother or the development of the child.

What can help you decide whether to have treatment?

If you have no symptoms and only slightly high TSH levels, you usually won't need treatment. Many doctors only recommend treatment if the TSH levels are very high (over 10 mU/L). Other factors may also play a role in the decision, such as how high your overall risk of cardiovascular disease is.

In people with high levels of thyroid antibodies (Hashimoto’s disease), treatment is sometimes already recommended at TSH levels of over 6 mU/L. That is done to prevent subclinical hypothyroidism from developing into overt hypothyroidism (with symptoms). There is hardly any research on whether treatment can achieve this goal.

The decision about whether to have treatment very much depends on your personal preferences because so many questions remain unanswered. Some people might prefer not to take any hormones unless it's absolutely necessary – even though thyroxine treatment is considered to be quite safe at the correct dose.

Others might want to try out treatment if they often feel exhausted or have constipation and think it's due to subclinical hypothyroidism. If the treatment doesn't make those problems go away, they are probably being caused by something else.

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IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

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