Deciding whether or not to treat subclinical hypothyroidism

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Subclinical (latent) hypothyroidism means that the thyroid gland is still producing enough thyroid hormones. But some blood values may suggest the early stages of a hormone deficiency. Experts don't agree on whether subclinical hypothyroidism should be treated. It's not clear in which cases treatment would have advantages.

If you already have noticeable (“overt” or “manifest”) hypothyroidism, the thyroid is no longer producing enough thyroid hormones. Thyroxine is the most important of the thyroid hormones. It helps regulate many of the body’s functions to balance your metabolism. Too little thyroxine can cause a number of different health problems. The symptoms range from cold hands to physical weakness, difficulty concentrating and depression. Hypothyroidism can be treated easily by taking a tablet containing the thyroid hormone once a day. These tablets act to replace the thyroxine that is not being produced. This usually makes the symptoms disappear completely.

Because it doesn't cause any symptoms, subclinical hypothyroidism isn't noticeable. The TSH (thyroid-stimulating hormone) value is too high, but the thyroid is still producing enough hormones. This hormone is produced in the pituitary gland and acts as a trigger for the thyroid to start producing the thyroid hormones. TSH levels that are just a little too high may be the first sign of the early stages of hypothyroidism: The pituitary gland responds to lower levels of thyroid hormones by increasing TSH production to activate the thyroid.

It is estimated that about 5 out of 100 people have subclinical hypothyroidism. Slightly elevated TSH levels are usually detected by accident during a routine examination. But taken on their own they don't pose any health risk. It's also possible that TSH levels are high only temporarily, for example after intense physical activity. Experts don't fully agree on how to decide when subclinical hypothyroidism should be treated.

How is subclinical hypothyroidism diagnosed?

Thyroid values like TSH are measured in blood tests. Because a single test can be misleading, a second test is usually done 2 or 3 months later. In both tests, the blood is taken at the same time of day because TSH levels can fluctuate over the course of 24 hours. Subclinical hypothyroidism is diagnosed when both TSH readings are high but the thyroid hormone thyroxine is still within the normal range.

Experts don't agree on which TSH levels should be considered too high. Some suggest that TSH levels of over 2.5 milliunits per liter (mU/L) are abnormal, while others consider levels of TSH to be too high only after they have reached 4 to 5 mU/L.

Both children and teenagers as well as older people have somewhat higher TSH levels than middle-aged people. Because of this, thyroid specialists have been debating whether a higher threshold should be used for these age groups. Being severely overweight and certain medications can also increase TSH. TSH levels are likely to fluctuate more during pregnancy.

How does subclinical hypothyroidism develop?

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

One study followed people with high levels of TSH over a period of two to three years. The participants didn't have any symptoms or diagnosed thyroid disorders. They were divided into three groups depending on how high their TSH levels were. The study produced the following results:

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

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

The probability that overt hypothyroidism develops from subclinical hypothyroidism is greater if the thyorid is enlarged and thyroid antibodies are detectable in the blood. And women generally have a higher risk than 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 detecting thyroid antibodies in your blood is not a sure sign that you have an underactive thyroid.

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 is some evidence suggesting that your risk of cardiovascular disease might slightly increase over the long term if TSH levels are higher than 10 mU/L. This link has not been observed for slightly elevated TSH levels lower than 10 mU/L.

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

There are no studies showing any advantages of treating subclinical hypothyroidism in children and teenagers. Sometimes young people have higher TSH levels because they are overweight, so treatment with medication is usually not a good idea.

Does thyroxine treatment have side effects?

No good-quality research is available on the side effects of treating subclinical hypothyroidism with thyroxine, but it's generally considered to be a well-tolerated drug. Because the body usually produces this hormone on its own, there are no problems if 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 or a racing heartbeat.

Does treatment make sense during pregnancy

Sometimes subclinical hypothyroidism in pregnant women is treated. For this purpose, they are occasionally offered a diagnostic screening test. In Germany this is provided as a “individual health care service” (IGeL Leistung).

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 levels of TSH or thyroid antibodies. The largest study yet doesn't show any advantage of treatment with thyroxine in pregnancy, neither for the risk of a premature birth or miscarriage, nor for the child's development.

Treatment: Yes or no?

People who have no symptoms and only slightly elevated TSH levels usually don't need treatment. Many doctors don't recommend treatment unless 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.

Treatment is sometimes recommended already starting at TSH levels of over 6 mU/L in people with high levels of thyorid antibodies (Hashimoto’s thyroiditis). That is done to prevent subclinical hypothyroidism from becoming overt hypothyroidism. There is hardly any research on whether treatment can achieve that goal.

Deciding whether to have treatment or not very much comes down to personal preference since so many questions are still unanswered. You might prefer to not take any hormones unless it is absolutely necessary – even though thyroxine treatment is considered to be quite safe when taken at the correct dose. Especially if you have no symptoms or just very mild symptoms and hardly notice any effect from the medication, it can be a challenge to keep taking the tablets on a daily basis.

Some people may believe things like exhaustion or constipation are symptoms of their subclinical hypothyroidism and because of this try out treatment - even though it's not the thyroid causing their problems. Instead, their symptoms may have a number of other causes.

If they don't go away with treatment, they are probably not being caused by an underactive thyroid. Then you could stop taking the medication after talking it over with your doctor.