Are there benefits to treating subclinical hypothyroidism?

High TSH levels might be detected during a routine blood test, for instance. TSH levels are also measured if certain treatments or examinations are planned, for example ones 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.

There is some disagreement among experts regarding which specific 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 still consider that to be normal.

Higher TSH levels are higher in older people, so doctors have raised the limits for older age groups. In middle age, for instance, a level of 4 mU/l is considered to be high. The level increases to 6 mU/l for people over the age of 80. Being severely overweight and using certain medications can also increase your TSH levels. TSH levels can go up and down a bit during pregnancy.

The way that subclinical hypothyroidism develops over time depends on a number of different factors – including the TSH levels: Slightly high TSH levels often return to normal on their own. But people who have very high levels 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 necessarily mean that hypothyroidism will definitely develop.

Some doctors will advise you to start treatment with hormone tablets (L-thyroxine) immediately if you have subclinical hypothyroidism. This is because there is some evidence suggesting that your risk of cardiovascular disease might slightly increase over the long term if TSH levels are moderately to very high. This link hasn't been observed for slightly high TSH levels.

But it is not clear whether L-thyroxine can lower the risk of complications or symptoms of hypothyroidism because the good-quality studies have not included enough people so far.

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.

No good-quality research is available on the side effects of treating subclinical hypothyroidism with thyroxine, but it's generally considered to be well tolerated. 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.

Some evidence suggests that subclinical hypothyroidism during pregnancy can increase the risk of miscarriage or premature birth. That is why a test is offered to pregnant women. The test is an individual healthcare service in Germany (individuelle Gesundheitsleistung, or IGeL for short) so it is not covered by statutory health insurers. It is also not clear whether treating pregnant women with L-thyroxine can lower the risks for the child, so experts do not recommend 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. Age also plays a role. People over the age of 75 years are not advised to use L-thyoxine unless their TSH level is 20 mU/l or higher. 10 mU/l is the limit for younger people. If your level is between 10 and 20 mU/l, you can have your TSH checked regularly and then start with treatment if it gets higher than 20 mU/l.

Other factors may also play a role in the decision, such as whether you already have a high risk of cardiovascular disease.

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.

Deciding whether or not 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 L-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.