Subclinical hypothyroidism is an early, mild form of hypothyroidism, a condition in which the body doesn’t produce enough thyroid hormones.

It’s called subclinical because only the serum level of thyroid-stimulating hormone from the front of the pituitary gland is a little bit above normal. The thyroid hormones produced by the thyroid gland are still within the laboratory’s normal range.

These hormones help support heart, brain, and metabolic functions. When thyroid hormones aren’t working properly, this affects the body.

According to published research, 3 to 8 percent of people have subclinical hypothyroidism. This condition can progress to full-blown hypothyroidism.

In one study, 26.8 percent of those with subclinical hypothyroidism developed full-blown hypothyroidism within 6 years of their initial diagnosis.

The pituitary gland, located at the base of the brain, secretes multiple hormones, including a substance called thyroid-stimulating hormone (TSH).

TSH triggers the thyroid, a butterfly-shaped gland at the front of the neck, to make the hormones T3 and T4. Subclinical hypothyroidism occurs when TSH levels are slightly elevated but T3 and T4 are normal.

Subclinical hypothyroidism and full-blown hypothyroidism share the same causes. These include:

  • a family history of autoimmune thyroid disease, such as Hashimoto’s thyroiditis (an autoimmune condition that harms thyroid cells)
  • injury to the thyroid (for example, having some abnormal thyroid tissue removed during head and neck surgery)
  • the use of radioactive iodine therapy, a treatment for hyperthyroidism (a condition when too much thyroid hormone is produced)
  • taking medications that contain lithium or iodine

A variety of things, most of which are outside of your control, increase the chances of developing subclinical hypothyroidism. These include:

  • Gender. A study published in the journal Endocrinology and Metabolism showed that women are more likely to develop subclinical hypothyroidism than men. The reasons aren’t entirely clear, but researchers suspect the female hormone estrogen may play a role.
  • Age. TSH tends to rise as you age, making subclinical hypothyroidism more prevalent in older adults.
  • Iodine intake. Subclinical hypothyroidism tends to be more prevalent in populations that consume sufficient or excess iodine, a trace mineral essential for proper thyroid function. It can help to be familiar with the signs and symptoms of an iodine deficiency.

Subclinical hypothyroidism most of the times has no symptoms. This is especially true when TSH levels are only mildly elevated. When symptoms do arise, however, they tend to be vague and general and include:

It’s important to note that these symptoms are nonspecific, meaning they can be present in individuals with normal thyroid function and not related to subclinical hypothyroidism.

Subclinical hypothyroidism is diagnosed with a blood test.

A person with a normal functioning thyroid should have a blood TSH reading within the normal reference range, which commonly goes up to 4.5 milli-international units per liter (mIU/L) or 5.0 mIU/L.

However, there’s debate underway in the medical community about lowering the highest normal threshold.

People with a TSH level above the normal range, who have normal thyroid gland hormone levels, are considered to have subclinical hypothyroidism.

Because amounts of TSH in the blood can fluctuate, the test may need to be repeated after a few months to see if the TSH level has normalized.

There’s a lot of debate about how — and even if — to treat those with subclinical hypothyroidism. This is especially true if TSH levels are lower than 10 mIU/L.

Because a higher TSH level can start to produce adverse effects on the body, people with a TSH level over 10 mIU/L are generally treated.

According to research from 2009, evidence is mostly inconclusive that those with TSH levels between 5.1 and 10 mIU/L will benefit from treatment.

In deciding whether or not to treat you, your doctor will take into consideration things like:

  • your TSH level
  • whether or not you have antithyroid antibodies in your blood and a goiter (both are indications the condition may progress to hypothyroidism)
  • your symptoms and how much they’re affecting your life
  • your age
  • your medical history

When treatment is used, levothyroxine (Levoxyl, Synthroid), a synthetic thyroid hormone taken orally, is often recommended and is generally well tolerated.

Heart disease

The connection between subclinical hypothyroidism and cardiovascular disease is still being debated. Some studies do suggest that elevated TSH levels, when left untreated, may contribute to developing the following:

  • high blood pressure
  • high cholesterol

In a 2005 study looking at older men and women, those with a blood TSH level of 7 mIU/L and above were at twice the risk or more for having congestive heart failure compared to those with a normal TSH level. But some other studies didn’t confirm this finding.

During pregnancy, a blood TSH level is considered elevated when it exceeds 2.5 mIU/L in the first trimester and 3.0 mIU/L in the second and third. Proper thyroid hormone levels are necessary for fetal brain and nervous system development.

Research published in The BMJ found that pregnant women with a TSH level between 4.1 and 10 mIU/L who were subsequently treated were less likely to have a miscarriage than their counterparts who weren’t treated.

Interestingly, though, women with a TSH level between 2.5 and 4 mIU/L didn’t see any reduced risk of pregnancy loss between those treated and those untreated if they had negative thyroid antibodies.

Assessing the status of antithyroid antibodies is important.

According to a 2014 study, women with subclinical hypothyroidism and positive antithyroid peroxidase (TPO) antibodies tend to have the highest risk of adverse pregnancy outcomes, and adverse outcomes happen at a lower TSH level than in women without TPO antibodies.

A 2017 systematic review found that the risk of pregnancy complications was apparent in TPO-positive women with a TSH level greater than 2.5 mU/L. This risk wasn’t consistently apparent in TPO-negative women until their TSH level exceeded 5 to 10 mU/L.

There’s no good scientific evidence that eating or not eating certain foods will definitely help to stave off subclinical hypothyroidism or treat it if you’ve already been diagnosed. It’s important, however, to get an optimal amount of iodine in your diet.

Too little iodine can lead to hypothyroidism. On the other hand, too much may lead to either hypothyroidism or hyperthyroidism. Good sources of iodine include iodized table salt, saltwater fish, dairy products, and eggs.

The National Institutes of Health recommends 150 micrograms per day for most adults and teenagers. One-quarter teaspoon of iodized salt or 1 cup of low-fat plain yogurt provides about 50 percent of your daily iodine needs.

All in all, the best thing you can do for your thyroid function is to eat a well-balanced, nutritious diet.

Because of conflicting studies, there’s still a lot of debate about how and if subclinical hypothyroidism should be treated. The best approach is an individual one.

Talk to your doctor about any symptoms, your medical history, and what your blood tests show. This handy discussion guide can help you get started. Study your options and decide on the best course of action together.