Thyroid stimulating hormone (TSH) tells the thyroid gland to produce hormones. Usually, high TSH levels indicate that the thyroid is underactive and that it needs more stimulation in order to work. Sometimes, however, people with hypothyroidism have normal TSH levels.

This can happen for several reasons. For instance, some people may have elevated TSH levels that are not high enough to meet the criteria for hypothyroidism.

Another possibility is that a person has a less common form of hypothyroidism called secondary, or central, hypothyroidism. This occurs when there is a problem with TSH rather than with the thyroid gland.

In this article, we will look in more detail at how and why a person with hypothyroidism might have normal TSH levels.

A red spirit level on a flat concrete surface, to represent TSH levels.Share on Pinterest
the_burtons/Getty Images

Yes, it is possible to have hypothyroidism and normal TSH levels in the blood.

Most people with hypothyroidism have high TSH because their thyroid gland is not releasing enough hormones. In response to this, the body produces more TSH in order to get the thyroid to work. This results in high levels of TSH in the blood, which a doctor can test for.

Healthcare professionals call this form of hypothyroidism primary hypothyroidism. It is the most common form of the condition. However, there are other types of hypothyroidism that do not always cause high TSH levels.

Additionally, TSH testing may not always be reliable. A 2018 study found that TSH levels could show hypothyroidism in many, but not all, people. Mass spectrometry to measure thyroid hormones was more effective.

The authors of the study recommend using more than just one test to assess thyroid function, especially in individuals who have symptoms that indicate hypothyroidism.

The normal range for TSH varies from laboratory to laboratory, but generally, it is 0.4–4 milliunits per liter (mU/l).

Levels of 4–10 mU/l are in the higher range. This usually indicates hypothyroidism, but not always. Illness or older age can also sometimes elevate TSH levels.

If TSH rises above 10 mU/l, doctors usually treat it as hypothyroidism, and a person will likely have symptoms.

A number of factors may cause TSH levels to be normal in a person with hypothyroidism, including:

Subclinical hypothyroidism

Subclinical hypothyroidism means that TSH numbers are elevated. Different laboratories give different numbers. Usually, however, the figure is between 4 and 10 mU/l.

Sometimes, a person has a TSH level that, technically, does not fall outside the healthy range but is higher than usual for that person. This can indicate that their thyroid is beginning to work less efficiently.

People with subclinical hypothyroidism can also have elevated thyroid antibodies, but typically, they do not have low levels of the thyroid hormones T3 or T4.

Doctors usually only treat subclinical hypothyroidism if an individual has symptoms.

However, once a person receives a diagnosis, they may need more frequent thyroid screenings. This is because subclinical hypothyroidism is a risk factor for developing hypothyroidism if an individual also has elevated thyroid antibodies.

Subclinical hypothyroidism in pregnancy

Pregnancy can be taxing on the thyroid for people with an underlying thyroid disease.

It can cause the body to release more TSH to keep the thyroid functioning as it should. As a result, some people may develop subclinical or overt hypothyroidism during pregnancy.

Thyroid disease in pregnancy may increase the risk of certain complications. For this reason, a doctor may suggest treating it, even if a person’s TSH is not high enough for an official hypothyroidism diagnosis.

Researchers recommend treatment when TSH levels rise above 4 mU/l in pregnancy.

Secondary hypothyroidism

Secondary, or central, hypothyroidism is a less common form of hypothyroidism. It occurs when the thyroid gland itself is healthy, but TSH is not working as it should.

The pituitary gland in the brain makes TSH. If it starts making less than usual, the thyroid will release fewer thyroid hormones.

Sometimes, the TSH that the pituitary gland does produce may be biologically inactive, meaning it has less impact on the thyroid.

Secondary hypothyroidism can also occur due to problems with the hypothalamus, which is a part of the brain responsible for making thyroid releasing hormone (TRH). TRH production stimulates TSH production.

People with secondary hypothyroidism may have normal or slightly low TSH levels. In this case, doctors will base a diagnosis on low T4 levels.

Some potential causes of secondary hypothyroidism include:

  • certain medications, such as opioids, prednisone, and dopamine
  • radiation therapy to the brain
  • TRH resistance or deficiency
  • Sheehan’s syndrome, which can occur when a person loses a lot of blood during childbirth, leading to the damage of the pituitary gland
  • lymphocytic hypophysitis, which is when a type of white blood cell infiltrates the pituitary gland
  • tumors on the pituitary gland or hypothalamus

Variations in testing

With hypothyroidism tests, there is no standardized threshold for what is normal and what is not. This can mean that some people fall into a “normal” range with one test provider and into a “high” range with another.

Most laboratories consider TSH levels of 10 mU/l or above to be high enough to start treatment.

The standard treatment for hypothyroidism is the medication levothyroxine. It works by replacing the thyroid hormones that the thyroid is not producing. For many, this relieves the symptoms and improves the quality of life.

The dosage will depend on how severe a person’s hypothyroidism is and how their body responds to treatment.

In most cases, a doctor will recommend a dosage of 1.6 micrograms per kilogram of body weight per day. It is possible, however, to adjust this if a person needs less or more.

Individuals with subclinical hypothyroidism may need relatively small doses of treatment, if they choose to treat at all.

People with secondary hypothyroidism may also require treatment to address the underlying issue that is affecting the hypothalamus or pituitary gland.

It is important to take thyroid hormone at a different time of day than one does other vitamins, especially iron supplements, since they can interfere with the body’s ability to absorb treatment.

There are other conditions that can cause similar symptoms to hypothyroidism, which could explain why someone has these symptoms but normal TSH levels.

If a doctor confirms the diagnosis of suspected hypothyroidism, they may also screen for:

People who have hypothyroid symptoms with normal TSH levels can consider asking a doctor the following questions:

  • Are my T3 and T4 levels also normal?
  • Can we retest in a few months?
  • What other conditions could explain my symptoms? Could we test for those?
  • Could any medications I take be causing my symptoms?
  • What are the next steps?

TSH instructs the thyroid to produce hormones that help regulate a person’s energy levels, body temperature, sleep, and more.

For many, high TSH levels indicate an underactive thyroid, but people can have the condition and still have TSH that falls into a normal range.

In subclinical hypothyroidism, TSH may only be slightly elevated. In cases of secondary hypothyroidism, TSH may be normal or slightly low.

To get an accurate picture of someone’s thyroid function, doctors should perform a series of tests to rule out or confirm suspected hypothyroidism. It may help to consult an endocrinologist, who can provide specialized guidance.