Subclinical hyperthyroidism is when thyroid-stimulating hormone (TSH) levels are low or undetectable, but thyroid hormone levels remain healthy. Usually, it is symptomless and may not need treatment.

Subclinical hyperthyroidism is when you have low levels of TSH but normal levels of T3 and T4.

T4 (thyroxine) is a major hormone secreted by your thyroid gland. T3 (triiodothyronine) is a modified version of T4. The amount of T4 produced by your thyroid gland is controlled by the levels of TSH production by your pituitary gland and vice versa.

Therefore, if your pituitary gland sees very little T4, it will produce more TSH to tell your thyroid gland to produce more T4. Once the amount of T4 reaches appropriate levels, your pituitary gland recognizes that and stops producing TSH.

In people with subclinical hyperthyroidism, the thyroid produces normal levels of T4 and T3. Yet, they have lower-than-normal TSH levels. This imbalance of hormones leads to the condition.

The prevalence of subclinical hyperthyroidism in the general population is estimated to range from 0.6 to 16 percent. It depends on the diagnostic criteria used.

Most people who have subclinical hyperthyroidism have no symptoms of an overactive thyroid. If symptoms of subclinical hyperthyroidism are present, they’re mild and nonspecific. These symptoms can include:

  • rapid heartbeat or heart palpitations
  • tremors, typically in your hands or fingers
  • sweating or intolerance to heat
  • nervousness, anxiety, or feeling irritable
  • weight loss
  • difficulty concentrating

Subclinical hyperthyroidism can be caused by both internal (endogenous) and external (exogenous) factors.

Internal causes of subclinical hyperthyroidism can include:

  • Graves’ disease. Graves’ disease is an autoimmune disorder that causes an overproduction of thyroid hormones.
  • Multinodular goiter. An enlarged thyroid gland is called a goiter. A multinodular goiter is an enlarged thyroid where multiple lumps, or nodules, can be observed.
  • Thyroiditis. Thyroiditis is an inflammation of the thyroid gland, which includes a group of disorders.
  • Thyroid adenoma. A thyroid adenoma is a benign tumor of the thyroid gland.

External causes of subclinical hyperthyroidism include:

  • excessive TSH-suppressive therapy
  • unintentional TSH suppression during hormone therapy for hypothyroidism

Subclinical hyperthyroidism can occur in pregnant women, particularly in the first trimester. However, it’s not associated with adverse pregnancy outcomes and typically doesn’t require treatment.

If your doctor suspects that you have subclinical hyperthyroidism, they’ll first assess your levels of TSH.

If your TSH levels come back low, your doctor will then evaluate your levels of T4 and T3 to see if they’re within the normal ranges.

In order to perform these tests, your doctor will need to take a sample of blood from your arm.

The normal reference range for TSH in adults is typically defined as 0.4 to 4.0 milli-international units per liter (mIU/L). However, it’s important to always refer to the reference ranges provided for you on the laboratory report.

Subclinical hyperthyroidism is generally classified into two categories:

  • Grade I: Low, but detectable TSH. People in this category have TSH levels between 0.1 and 0.4 mlU/L.
  • Grade II: Undetectable TSH. People in this category have TSH levels less than 0.1 mlU/L.

When subclinical hyperthyroidism is left untreated, it can have several negative effects on the body:

  • Increased risk of hyperthyroidism. People who have undetectable TSH levels are at an increased risk for developing hyperthyroidism.
  • Negative cardiovascular effects. People who are untreated can develop:

A review of scientific literature found that low TSH levels spontaneously returned to normal in 50 percent of people with subclinical hyperthyroidism.

Whether the condition needs treatment depends on:

  • the cause
  • how severe it is
  • the presence of any related complications

Treatment based on the cause

Your doctor will work to diagnose what may be causing your subclinical hyperthyroidism. Determining the cause can help determine the appropriate treatment.

Treating internal causes of subclinical hyperthyroidism

If you have subclinical hyperthyroidism due to Graves’ disease, medical treatment is required. Your doctor will likely prescribe radioactive iodine therapy or anti-thyroid medications, such as methimazole.

Radioactive iodine therapy and anti-thyroid medications can also be used to treat subclinical hyperthyroidism due to multinodular goiter or thyroid adenoma.

Subclinical hyperthyroidism due to thyroiditis typically resolves spontaneously without any additional treatment required. If thyroiditis is severe, your doctor may prescribe anti-inflammatory drugs. These could include nonsteroidal anti-inflammatory drugs (NSAIDs) or corticosteroids.

Treating external causes of subclinical hyperthyroidism

If the cause is due to TSH-suppressive therapy or hormone therapy, your doctor may adjust the dosage of these drugs where appropriate.

Treatment based on severity

If your TSH levels are low but still detectable and you don’t have complications, you may not receive immediate treatment. Instead, your doctor may choose to retest your TSH levels every few months until they return to normal or your doctor is satisfied that your condition is stable.

Treatment may be required if your TSH levels fall into Grade I or Grade II and you’re in the following risk groups:

  • you’re over 65 years of age
  • you have cardiovascular disease
  • you have osteoporosis
  • you have symptoms suggestive of hyperthyroidism

Your treatment will depend on what type of condition is causing your subclinical hyperthyroidism.

Treatment with the presence of complications

If you’re experiencing cardiovascular or bone-related symptoms due to your subclinical hyperthyroidism, you may benefit from beta-blockers and bisphosphonates.

Some studies have shown that the negative effects on bone density can be relieved by making sure that you get an adequate daily dose of calcium.

You may have some weight loss if you have subclinical hyperthyroidism. This is because people with an overactive thyroid have an elevated basal metabolic rate (BMR). The calorie requirements for maintaining your weight will be higher.

Subclinical hyperthyroidism is when you have low levels of TSH but have normal levels of T3 and T4. If you’re experiencing symptoms of subclinical hyperthyroidism, your doctor can use a series of blood tests to come to a diagnosis.

Since this condition can be caused by a variety of different conditions, the treatment you receive will depend on the cause and severity. Once your levels return to normal either naturally or through use of medication, your outlook should be excellent.