If You Have a Borderline Underactive Thyroid, Do You Need To Treat It?

What to know before getting treatment for a borderline underactive thyroid, according to Health's resident medical editor.

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During your annual doctor's visit, you might have to complete routine labs where they will draw blood to check for various health conditions such as cholesterol and blood glucose. If your thyroid stimulating hormones (TSH levels) are borderline high your doctor may want to talk about treatment options for your thyroid.

Your thyroid gland governs much of the metabolic activity in your body, and when it's underactive (meaning it's not producing enough thyroid hormone), that can cause all sorts of symptoms, including weight gain, feeling cold, and fatigue. However, sometimes hypothyroidism is so minor that there are only very mild, nonspecific symptoms, or none at all, and it's detectable only through blood tests.

In order to diagnose you with hypothyroidism, a healthcare provider will order a blood test to measure the levels of two hormones: thyroxine (T4) and thyroid-stimulating hormone (TSH). TSH is produced by your brain's pituitary gland and regulates how much T3 (another thyroid hormone) and T4 your thyroid releases.

When you have an underactive thyroid, as is the case with hypothyroidism, your brain releases more TSH in an effort to increase thyroid hormone levels. This is why high levels of TSH (above 5.9 mIU/L) and low levels of T4 (below 5.0 μg/dL) indicate an underactive thyroid, which in turn results in a hypothyroidism diagnosis, per UCLA Health.

In the case of a borderline underactive thyroid, your TSH levels are high but your T4 reading is relatively normal (that is, between 0.5 and 5.0 mIU/L). This is also called subclinical hypothyroidism. It's an early form of hypothyroidism and it affects up to 20% of adults, according to a 2019 review published in the Cleveland Clinic Journal of Medicine.

In 70% of cases, subclinical hypothyroidism is asymptomatic (meaning you won't have symptoms). Still, this subclinical form increases a person's chances of developing hypothyroidism later in life. In addition, hypothyroidism and even subclinical hypothyroidism can impact reproductive function. Both conditions have been linked with an increased risk of infertility, miscarriage, and pregnancy complications.

How To Treat Hypothyroidism

To treat hypothyroidism, a patient needs to take hormone medication to replace the amount that the thyroid can't make. According to the American Thyroid Association (ATA), the goal of thyroid hormone treatment is to closely replicate normal thyroid functioning.

Thyroid hormone is available as levothyroxine, which is biologically equivalent to your own thyroid hormone, thyroxine (T4). It is most commonly prescribed in tablet form but is now also available in gel capsule or liquid forms.

Because borderline cases can progress to overt hypothyroidism, some healthcare providers say it's useful to take medication. This is especially true if you also have a risk of heart disease as even mild hypothyroidism can lead to elevated cholesterol levels.

Balancing Benefits and Risks

That said, other research suggests that taking medication for subclinical hypothyroidism—another name for a borderline underactive thyroid) may have no benefit.

According to a 2021 analysis presented in JAMA Internal Medicine, levothyroxine is one of the most commonly prescribed drugs in the US, with approximately 7% of the population estimated to have an active prescription. The authors conducted a retrospective analysis of insurance claims data from 2008 and 2018.

They found that levothyroxine treatment was commonly initiated for mildly increased TSH levels and that 60% of patients started levothyroxine for the treatment of subclinical hypothyroidism. Based on their findings, the authors write, "for nonpregnant adults with subclinical hypothyroidism, evidence consistently demonstrates no clinically relevant benefits of levothyroxine replacement for quality of life or thyroid-related symptoms."

And a 2014 study of patients in the United Kingdom published by JAMA Internal Medicine suggests the widespread prescribing of levothyroxine to boost thyroid function among patients with borderline high levels of thyroid-stimulating hormone may indicate overtreatment.

The authors highlight that the ATA guidelines recommend considering levothyroxine therapy at thyrotropin levels of 10 mIU/L or less when a patient meets any of the following criteria:

  • There are clear symptoms of hypothyroidism.
  • A patient has positive thyroid autoantibodies.
  • There is evidence of atherosclerotic cardiovascular disease (hardening of the arteries) or heart failure.

Treatment of patients with thyrotropin levels at or below 10 mIU/L without symptoms may cause more harm than good and may represent overtreatment.

If you're not experiencing any symptoms, you should think about whether the potential side effects (such as loss of bone density) are worth it. Your healthcare provider or an endocrinologist can help you weigh the costs and benefits of treatment in your case. Your provider may have you get regular tests to monitor your levels and hold off on taking medication. Or, if you have symptoms, they may advise taking the medication temporarily to see whether you start to feel better.

Health's medical editor, Roshini Rajapaksa, MD, is an assistant professor of medicine at the NYU School of Medicine.

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