Subclinical Hyperthyroidism

What is subclinical hyperthyroidism?

Subclinical hyperthyroidism happens when you have a low or undetectable thyroid-stimulating hormone (TSH) level with normal thyroxine (T4) and triiodothyronine (T3) levels.

You don’t technically have hyperthyroidism (commonly called overt hyperthyroidism, in comparison), but it has the potential to develop into overt hyperthyroidism.

Hyperthyroidism happens when your thyroid produces excessive amounts of thyroid hormones (thyroxine and triiodothyronine). “Subclinical” describes a condition that’s not severe enough to cause definite symptoms.

Thyroid-stimulating hormone, commonly called TSH and also referred to as thyrotropin, is a hormone that your pituitary gland releases to trigger your thyroid to produce and release its own hormones — thyroxine (T4) and triiodothyronine (T3). These two hormones are essential for maintaining your body’s metabolism — how your body transforms the food you eat into energy and uses it.

Subclinical hyperthyroidism is often temporary but can be long-lasting. It may or may not require treatment.

Who does subclinical hyperthyroidism affect?

Anyone can have subclinical hyperthyroidism, but it most commonly affects people receiving thyroid hormone replacement therapy for hypothyroidism and people over the age of 65.

How common is subclinical hyperthyroidism?

Subclinical hyperthyroidism is uncommon in the United States.

Approximately 0.7% of the U.S. population has subclinical hyperthyroidism with thyroid-stimulating hormone (TSH) levels less than 0.1 mIU/ L (milli-international units per liter of blood) and about 1.8% of the population has levels less than 0.4 mIU per L.

However, in countries and regions where iodine deficiency is widespread, the prevalence of subclinical hyperthyroidism can be as high as 15% in people over the age of 70.

What are the symptoms of subclinical hyperthyroidism?

Most of the time, subclinical hyperthyroidism doesn’t cause any symptoms (it’s asymptomatic).

However, it can sometimes present with mild symptoms of hyperthyroidism, which include:

• Rapid heartbeat (palpitations).
• Feeling shaky and/or nervous.
• Weight loss.
• Increased appetite.
• Diarrhea and more frequent bowel movements.
• Thin, warm and moist skin.
• Menstrual changes.

What causes subclinical hyperthyroidism?

Normally, multiple hormones and glands in your endocrine system work together to carefully control the level of TSH in your bloodstream through a feedback loop.

To start, your hypothalamus releases thyroid-releasing hormone (TRH) to trigger the release of thyroid-stimulating hormone (TSH) by your pituitary gland.

TSH then stimulates cells in your thyroid to release thyroxine or T4 (80%) and triiodothyronine or T3 (20%) into your bloodstream. These two hormones prevent your pituitary gland from producing more TSH if the levels of thyroxine and triiodothyronine are too high, thus completing the cycle. When T4 and T3 levels drop, the cycle starts over again.

However, in subclinical hyperthyroidism, due to a variety of possible thyroid issues, thyroid hormonal output doesn’t decrease like it normally should in response to the low TSH levels. This leads to low TSH levels and normal thyroxine (T4) and triiodothyronine (T3) levels, resulting in subclinical hyperthyroidism.

In general, the causes of subclinical hyperthyroidism are the same as the causes of overt hyperthyroidism. The most common causes of subclinical hyperthyroidism include:

• Overtreatment of thyroid hormone replacement therapy (levothyroxine) in people with hypothyroidism (most common cause).
• Multinodular toxic goiter (noncancerous growths on your thyroid that produce too much thyroid hormone).
• Graves’ disease (an autoimmune condition that results in the overproduction of thyroid hormones).
• Inflammation of your thyroid, which leads to a temporary increase in thyroid hormone levels.

How is subclinical hyperthyroidism diagnosed?

The diagnosis of subclinical hyperthyroidism is solely based on thyroid function testing (thyroid blood tests).

The normal test range for thyroid-stimulating hormone (TSH or thyrotropin) for a non-pregnant adult is 0.4 to 4.5 mIU/L (milli-international units per liter of blood).

If you had thyroid blood tests and the results indicate that your TSH levels are low or undetectable (0.1 to 0.4 mIU/L) and your thyroxine (T4) and triiodothyronine (T3) levels are in the normal range, it means you have subclinical hyperthyroidism.

Subclinical hyperthyroidism can be divided into two categories:

• Mild: Low but detectable TSH level — usually 0.1 to 0.4 mIU/L. This applies to 65% to 75% of people with subclinical hyperthyroidism.
• Severe: TSH level is less than 0.1 mIU/L. This applies to 25% to 35% of people with subclinical hyperthyroidism.

Does subclinical hyperthyroidism need to be treated?

Healthcare providers disagree on whether subclinical hyperthyroidism needs to be treated due to a lack of studies showing its effectiveness.

For most people with subclinical hyperthyroidism, providers recommend that they take a “wait and see” approach and not start treatment to see if the subclinical hyperthyroidism resolves on its own. However, providers may recommend treatment for people with TSH levels persistently less than 0.1 mIU/L if they:

• Are 65 years or older.
• Are younger than 65 years and have heart disease, osteoporosis and/or symptoms of hyperthyroidism.
• Are postmenopausal, younger than 65 years and not taking estrogen or bisphosphonates (a group of medicines used to treat bone problems).

Treatment is generally not required for subclinical hyperthyroidism in pregnancy.

What is the treatment for subclinical hyperthyroidism?

The treatment for subclinical hyperthyroidism — if your healthcare provider recommends it — depends on what’s causing it.

If you have toxic multinodular goiter or a single nodule on your thyroid, treatment is often radioactive iodine. This is an oral medication that your overactive thyroid cells absorb. The radioactive iodine damages these cells and causes your thyroid to shrink and thyroid hormone levels to go down over a few weeks.

If you have Graves’ disease, antithyroid drugs and radioactive iodine are usually the preferred treatment. The antithyroid drugs methimazole (Tapazole) and propylthiouracil (PTU) block the ability of your thyroid to make hormones.

Can I prevent subclinical hyperthyroidism?

In most cases, there’s nothing you can do to prevent subclinical hyperthyroidism or overt hyperthyroidism.

However, if you’re not getting enough iodine in your diet (or are consuming too much), you can develop subclinical or overt hyperthyroidism due to toxic goiter. While this is uncommon in the United States due to the use of iodized table salt, it can happen in countries or regions that have a widespread iodine deficiency.

What can I expect if I have subclinical hyperthyroidism?

Subclinical hyperthyroidism rarely progresses to overt hyperthyroidism, though the chance of progression is higher in people with very low TSH levels (less than 0.1 mIU/L).

Due to the various possible causes, each case of subclinical hyperthyroidism is different.

Even without progression to overt hyperthyroidism, subclinical hyperthyroidism can be associated with the following conditions:

• Atrial fibrillation.
• Heart failure.
• Coronary heart disease.
• Bone loss and fractures.
• Dementia.

People older than 65 years of age with severe subclinical hyperthyroidism are especially at risk.

If you’re concerned about these risks and other cardiovascular risk factors, talk to your healthcare provider.

When should I see my healthcare provider about subclinical hyperthyroidism?

In most cases, healthcare providers take a “wait and see” approach if your test results indicate you have subclinical hyperthyroidism.

If you start to experience symptoms of hyperthyroidism, such as feeling nervous or anxious and heart palpitations, talk to your provider. They’ll likely order another thyroid blood test to see if you have overt hyperthyroidism.

A note from Cleveland Clinic

Due to a lack of studies, there’s still a lot of debate about if subclinical hyperthyroidism should be treated. Each case and person is unique, so the best strategy is to talk to your healthcare provider about your concerns and options. They’re available to help you.