Your thyroid plays an important role in your overall health, influencing everything from your metabolism to your mood. Abnormal levels of thyroid hormones can lead to many health problems, from fatigue and joint pain to fertility issues and heart disease. In other words, that tiny thyroid gland at the base of your neck has a big job.
If you’ve been diagnosed with subclinical hypothyroidism — an early form of hypothyroidism or an underactive thyroid — it means that your thyroid hormone levels are somewhat abnormal, but not enough to qualify as hypothyroidism. The condition affects 4.3% of the US population, according to a review article in the Journal of the American Academy of Physician Assistants, and is more common in older people than in younger people.
How Thyroid Hormones Work
To better understand subclinical hypothyroidism (sometimes called mild hypothyroidism or pre-hypothyroidism), a quick science lesson should help: your thyroid gland, your pituitary gland (in your brain), and your hypothalamus (also in your brain) work all together as a kind of thyroid control panel. Together they produce what is called thyroid-stimulating hormone (TSH). This hormone is responsible for “stimulating” the production of two main thyroid hormones, triiodothyronine (called T3) and thyroxine (called T4).
When TSH is released, it triggers the production of T3 or T4 by the thyroid. When there is not enough T3 or T4 circulating in the body, more TSH is released; when there is enough or too much thyroid hormone circulating, the pituitary gland releases less TSH. It’s a finely tuned feedback loop.
Hypothyroidism or subclinical hypothyroidism occurs when something goes wrong with this system and the thyroid stops producing enough thyroid hormone to meet the body’s needs.
Subclinical symptoms of hypothyroidism
If your hormone levels are only mildly abnormal, you may not have any obvious symptoms. If you have symptoms, they may be vague or mimic other health conditions. As with complete hypothyroidism, symptoms you may experience include:
Depression or other mood issues
Feeling cold even though you are dressed warmly
Bowel problems, such as constipation
An enlarged thyroid gland, or goiter, which may appear as a lump or swelling in the front of the neck
Learn more about hypothyroidism symptoms.
Subclinical causes of hypothyroidism
The same culprits behind full-blown hypothyroidism can cause subclinical hypothyroidism. These include:
Hashimoto’s thyroiditis is one of the most common causes of hypothyroidism. This autoimmune disease occurs when your immune system attacks body tissues, especially the thyroid gland. If you have a family history of autoimmune thyroid disease, this increases your risk. Environmental factors can also trigger autoimmune disease, but researchers aren’t sure exactly how.
Certain medications can impact your thyroid function, including certain steroids, neurological medications, and pain relievers.
Treatment of hyperthyroidism or head or neck cancer
If you have ever been treated for hyperthyroidism (overactive thyroid), it is possible that excessive treatment with radioactive iodine or antithyroid medication could have led to hypothyroidism or subclinical hypothyroidism. Radiation therapy for head or neck cancer can also impact the thyroid.
Treatment of some thyroid conditions, including thyroid cancer and thyroid nodules, sometimes requires the surgical removal of part or all of the thyroid gland, called a thyroidectomy. Depending on how much of the gland is removed (and therefore how many thyroid hormone-producing cells remain in the body), thyroidectomy can result in subclinical or overt hypothyroidism.
In addition to these direct causes, several risk factors for subclinical hypothyroidism have been identified. According to The Mayo Clinicyou have a higher chance of developing subclinical hypothyroidism if you:
Diagnose subclinical hypothyroidism
“Subclinical hypothyroidism occurs when FT4 [free T4] is within normal range but TSH is elevated. FT4 is a form of T4 that circulates in the body and can freely enter body tissues as needed.
This is why the condition is called subclinical; thyroid hormones are within normal range. Since only the TSH is elevated, the problem does not meet the criteria for a definitive thyroid disorder.
As sick as a patient may feel, symptoms alone are not enough to make a diagnosis of subclinical hypothyroidism, Shah says. That’s why blood tests are essential: “It’s really the lab results that give us the answer.”
Most labs say that a normal serum TSH level is between 0.4 and 4.0 mIU/L, and a TSH level of 10 mIU/L or higher indicates hypothyroidism. A TSH level of 4.5 to 10 mIU/L is considered indicative of subclinical hypothyroidism.
Thyroid hormone reference ranges
You may notice that what constitutes a “normal” reference range for TSH and other thyroid hormone levels differs depending on the source. Not all labs or doctors agree on what “normal” or “abnormal” looks like.
Experts have debated a lot about whether the normal TSH reference range should stay the same or be made even narrower. For example, some research suggests — and some experts believe — that the “normal” TSH range should be reduced to 0.4 to 2.5 instead of 0.4 to 4.0.
Subclinical treatment of hypothyroidism
Medical practitioners have described the question of whether and how to treat subclinical hypothyroidism as controversial. According to Journal of the American Academy of Physician Assistants article noted above.
For this reason, Shah says, “Not everyone with SCH should be treated. Almost all patients with TSH levels above 10 should be treated, and most patients with TSH levels below 4 .5 should not be treated except in cases of infertility or attempted pregnancy (more on this below).
But what about the gray area between 4.5 and 10? This is where other factors come into play, explains Shah. For these patients,[treatment] really depends on other risk factors, such as age, symptoms, and the presence or absence of cardiovascular disease.
A TSH above 10 mIU/L has been linked to an increased risk of coronary heart disease and heart failure, so you’ll probably want to seek treatment if your TSH is near (but still below) this upper limit.
If your doctor decides to prescribe medication, “T4-only products, such as levothyroxine, are recommended for first-line treatment,” Shah says. “Generally, we start at the lowest dose possible to avoid overtreatment, especially in the elderly or those with underlying heart disease.”
Shah says your TSH levels should be checked again 6 to 12 weeks after starting levothyroxine treatment. From there, your healthcare provider will adjust your prescription as needed. Going forward, “Labs should be repeated about six months to assess stability, then every year thereafter if the TSH level is stable,” Shah says.
Can subclinical hypothyroidism turn into complete hypothyroidism?
While this concern is entirely valid, subclinical hypothyroidism is not always a definitive pathway to hypothyroidism, even if left untreated. In fact, a small, older study in The Journal of Clinical Endocrinology and Metabolism found that only about 27% of people over the age of 55 with subclinical hypothyroidism developed overt hypothyroidism.
Another study, in JAMA internal medicine, followed 422,242 people with subclinical hypothyroidism who were not treated with specific thyroid medications and found that a whopping 62% had normal TSH levels within five years. In other words, their subclinical hypothyroidism got better on its own over time, without medical intervention. This study classified subclinical hypothyroidism as a TSH level of 5.5 to 10 mIU/L.
Subclinical hypothyroidism and infertility
Thyroid hormones T3 and T4 play a key role in reproductive health, and thyroid dysfunction is a common cause of infertility. For this reason, Shah explains, you might consider treatment for a TSH level below 4.5 mIU/L if you’re trying to get pregnant or have infertility (when there’s really no reason to treat a TSH level as close to the threshold of normal, which is 4.0).
Related: Subclinical hypothyroidism: what is it? And could it affect fertility?
the American Thyroid Association (ATA) recommends that women of childbearing age with TSH levels between 2.5 and 10 be tested for antibodies to thyroperoxidase (TPO), which are common in people with hypothyroidism and are an indicator of disease autoimmune thyroid.
If anti-TPO antibodies are positive, treatment should be considered even if the TSH level is between 2.5 and 4.0, which would generally be considered “normal”.
If no antibodies to TPO are found, whether or not to seek treatment may depend on other health and risk factors. It’s important to work closely with a reproductive endocrinologist to make the best decisions for you.
The good news is that research has shown that treating hypothyroidism (subclinical or overt) often effectively treats infertility. In a study in the International Journal of Applied and Basic Medical Research, among women who were infertile and had high TSH levels, more than three-quarters were able to become pregnant within a year of starting hypothyroid therapy.
Questions to ask your doctor about subclinical hypothyroidism
Since thyroid levels can be confusing and aren’t universally recognized, it’s important to ask your doctor any questions you may have. These may include:
What thyroid level reference range do you use and is it up to date?
Would you consider my TSH level to be slightly elevated or closer to full-blown hypothyroidism?
Are all my other thyroid hormone levels normal?
What can I do to better manage my thyroid health?
How often should I come and test my TSH levels?
Are there other lab tests I should have to rule out other conditions that could explain my symptoms?