Subclinical hypothyroidism involves high levels of thyroid-stimulating hormone and typical thyroxine levels in the blood. This may be due to an autoimmune disease, a medication, or thyroid surgery. People with this condition may not experience symptoms.

While thyroid hormone levels in a person’s blood will be within the acceptable range, they may be slightly lower than expected.

Treatment for subclinical hypothyroidism may include the thyroid hormone replacement medication levothyroxine.

This article explores subclinical hypothyroidism, including the causes, symptoms, diagnosis, and treatments.

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Subclinical hypothyroidism is an early, mild form of hypothyroidism. It involves a hormone called thyroid stimulating hormone (TSH).

A person’s pituitary gland produces TSH, which, in turn, stimulates the thyroid gland, a butterfly-shaped organ at the base of the neck. The TSH causes the thyroid to produce thyroid hormones called triiodothyronine (T3) and thyroxine (T4).

Thyroid hormones regulate metabolic processes in the body, including heart rate, digestive function, and brain development.

Subclinical hypothyroidism occurs when T4 levels are normal but TSH levels are high.

Healthcare experts call it subclinical hypothyroidism because the free thyroid hormone levels are still normal. Some people do not experience symptoms; however, others can have the same symptoms that overt hypothyroidism would cause.

If the thyroid is not producing enough T4 to meet the body’s needs, TSH levels will spike to overwork the thyroid.

The higher the blood TSH level, the worse the thyroid function, and vice versa. A TSH level within a range of 5–10 milli-international units per liter (mIU/L) is a mild elevation, and levels above 10 mIU/L show a more severe degree of thyroid malfunction.

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Subclinical hypothyroidism can be asymptomatic most of the time. However, it can cause the same symptoms as overt hypothyroidism, including:

Subclinical hypothyroidism has several potential causes, including certain autoimmune conditions, surgery, and some medications.

Hashimoto’s thyroiditis, or Hashimoto’s disease

Hashimoto’s thyroiditis is an autoimmune disease in which a person’s immune system attacks the thyroid. Eventually, the body destroys its own healthy thyroid cells.

Hashimoto’s thyroiditis is the most common cause of subclinical hypothyroidism. According to a 2019 review, it accounts for 60–80% of cases.

Symptoms may include:

  • thyroid gland enlargement, which can cause pain and trouble swallowing
  • fatigue and sluggishness
  • weight gain
  • muscle weakness
  • depression
  • changes to the skin and hair

Thyroid surgery

Treatment for certain thyroid conditions, such as thyroid nodules and thyroid cancer, may involve the surgical removal of part or all of the thyroid. This procedure is known as a thyroidectomy.

Thyroid surgery may lead to subclinical hypothyroidism depending on the extent of removal of thyroid-producing cells from the thyroid.

Injury to the thyroid gland

Blunt force trauma or injury to the head or neck may reduce the thyroid’s ability to produce enough thyroid hormones to meet the body’s needs. This may lead to subclinical hypothyroidism.

Hyperthyroidism treatment

Radioactive iodine or antithyroid medications may cause subclinical hypothyroidism in people who have previously undergone treatment for hyperthyroidism.

Also, receiving radiotherapy treatment in the head or neck area may affect the thyroid and cause subclinical hypothyroidism.

Certain medications

According to a review published in the Cleveland Clinic Journal of Medicine, the following medications can affect thyroid function and cause subclinical hypothyroidism:

  • iodinated contrast
  • amiodarone
  • lithium
  • tyrosine kinase inhibitors
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Learn more about the thyroid and other conditions.

The following factors may increase a person’s chances of developing subclinical hypothyroidism:

  • Age: TSH levels increase as people age. People aged 65–70 years may have elevated levels of TSH and low T4 levels.
  • Sex: Subclinical hypothyroidism is more common among females than among males. Researchers don’t fully understand why this is the case, but they believe that the female hormone estrogen may be responsible.
  • Race: According to a 2019 review, subclinical hypothyroidism is more common in white people. The authors note that this may be due to a genetic factor.
  • Iodine intake: Populations that consume excess iodine may be at a higher risk of subclinical hypothyroidism than those that do not.

Before diagnosing this condition, a doctor will rule out non-thyroidal factors that can elevate TSH levels.

Health experts recommend that doctors assess a person’s TSH and T4 levels every 2–3 months.

To make a proper diagnosis, the doctor can order a combination of the following tests:

  • Thyroid function test: This can show the doctor how well a person’s thyroid gland is working. High TSH levels and normal T4 levels suggest subclinical hypothyroidism.
  • Thyroid peroxidase antibody (TPO) test: This can help the doctor measure the level of antibodies against blood TPO. Elevated TPO antibodies may indicate Hashimoto’s thyroiditis.
  • Ultrasound: An ultrasound can reveal the state and shape of the thyroid. Dark gray patches on an ultrasound may indicate hypothyroidism.

Doctors treat subclinical hypothyroidism with a replacement therapy known as levothyroxine therapy.

The American Thyroid Association (ATA) and the American Association of Clinical Endocrinology recommend starting levothyroxine therapy if a person meets the following conditions:

  • TSH higher than 10 mIU/L
  • presence of hypothyroid symptoms
  • presence of cardiovascular risk factors
  • positive TPO antibody

For people without a cardiovascular disease, the starting dose of levothyroxine is 1.6 mcg/kg. For people with cardiovascular disease, the starting dose is 25 mcg/kg, and a doctor will increase it as needed.

In the first 3 months of pregnancy, the fetus depends on the pregnant person’s thyroid hormones for brain and nervous system development. A lack of thyroid hormone during pregnancy may lead to adverse developmental outcomes, as well as complications for the pregnant person.

The ATA notes that pregnant people should take their thyroxine dose as instructed, as it is safe for them to do so.

In fact, most pregnant people need a higher dose of levothyroxine to keep up with the high thyroxine needs of their body and their developing baby.

If a pregnant person has received a diagnosis of subclinical hypothyroidism, they must start treatment immediately and continue it until they deliver.

A 2017 study found that thyroid hormone treatment improved the outcome in pregnant people whose TSH levels were 4.1–10 before treatment. This means that those who received treatment for thyroid problems had decreased pregnancy loss.

Subclinical hypothyroidism has an incidence rate of 3–15%, depending on the population.

Without treatment, it can escalate into full hypothyroidism. The risk of subclinical hypothyroidism progressing to hypothyroidism is 2–6% per year.

In hypothyroidism, the thyroid produces low levels of thyroid hormones.

Research from 2019 suggests that people with hypothyroidism tend to have high blood pressure and higher cholesterol levels. Research links both of these conditions to an increased risk of heart attack and stroke.

The ATA notes that hypothyroidism’s symptoms will likely develop slowly over a few months or years. However, some people develop symptoms quickly over a few months. Generally, the lower a person’s thyroid hormone levels and the longer they stay low, the more severe the symptoms will be.

If a person is experiencing even mild symptoms of subclinical hypothyroidism, they should consult a doctor.

A doctor will evaluate a person’s symptoms and determine the best treatment.

Subclinical hypothyroidism is a mild endocrine disorder characterized by normal T4 but high TSH levels.

The condition can be asymptomatic. This means that people with the condition do not know they have it. At other times, it can cause mild or severe symptoms.

Subclinical hypothyroidism can be a result of the autoimmune disease Hashimoto’s thyroiditis, certain medications, or other factors.

To accurately diagnose the condition, a doctor may order a thyroid function test, a TPO antibody test, and an ultrasound.

Levothyroxine therapy is the first line of treatment for people with subclinical hypothyroidism. It can help relieve symptoms and prevent the condition from progressing.