Subclinical hyperthyroidism is diagnosed through low or undetectable thyroid-stimulating hormone (TSH) levels, as thyroid hormone levels are within the normal range. Usually, this condition is symptomless and may not need treatment.

Hyperthyroidism happens when your thyroid gland overproduces hormones T3 and T4. Your pituitary gland detects this and reduces thyroid-stimulating hormone production in an attempt to reduce T3 and T4 levels.

Doctors can diagnose hyperthyroidism by finding high T3, T4, and low TSH levels.

Subclinical hyperthyroidism occurs when your levels of T3 and T4 are not high enough to be detected as abnormal, but you have low levels of TSH, indicating that your body is actively reducing T3 and T4 production.

Subclinical thyroidism can be considered a mild form of hyperthyroidism.

  • T4 (thyroxine) is a major hormone secreted by your thyroid gland. It is inactive.
  • T3 (triiodothyronine) is the active version of T4. Your body converts T4 into T3.
  • TSH is produced by your pituitary gland, which controls T4 production. If the pituitary gland detects low levels of T4, it increases the production of TSH, which increases the release of T4 by your thyroid gland. If the pituitary gland detects high T4 levels, it will reduce TSH production, which, in turn, lowers T4 levels.

Most people with subclinical hyperthyroidism have no symptoms of an overactive thyroid. If you do have signs of subclinical hyperthyroidism, they’re typically 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

If you think you may be experiencing symptoms of subclinical hyperthyroidism, speak with your doctor.

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

Internal causes

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 gland with multiple lumps, or nodules.
  • 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

External causes of subclinical hyperthyroidism include:

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

Subclinical hyperthyroidism can occur in pregnant people, particularly in the first trimester. However, it’s not associated with negative 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 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), according to the American Thyroid Association. However, it’s important to always refer to the reference ranges provided on your laboratory report.

Subclinical hyperthyroidism is generally classified into two categories:

  • Grade I: Low but detectable TSH. Levels range from 0.1 to 0.4 mlU/L.
  • Grade II: Undetectable TSH. Levels are less than 0.1 mlU/L.

A 2018 review of scientific literature found that low TSH levels often return to normal without any intervention.

Sometimes, subclinical hyperthyroidism may lead to:

  • Increased risk of hyperthyroidism. People who have undetectable TSH levels are at an increased risk of developing hyperthyroidism.
  • Negative cardiovascular effects. People with untreated subclinical hyperthyroidism can develop:
  • Decreased bone density. Untreated subclinical hyperthyroidism can lead to a decrease in bone density in postmenopausal women.

Whether the condition needs treatment depends on:

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

Treating internal causes

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

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 the type of condition that is causing your subclinical hyperthyroidism.

Treatment with 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 experience some weight loss if you have subclinical hyperthyroidism. This is because people with an overactive thyroid have an elevated basal metabolic rate (BMR). This means your calorie requirements to maintain your weight will be higher.

Subclinical hyperthyroidism is diagnosed 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 check if you have the condition.

Since this condition can be caused by various factors, the treatment you receive will depend on the cause and severity of your condition. Once your levels of TSH return to normal, either naturally or through medication, you’ll typically have no lasting effects of subclinical hyperthyroidism.