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What is Subclinical Hyperthyroidism?

Learn about subclinical hyperthyroidism and why those with hypothyroidism are at risk for developing it.
What is Subclinical Hyperthyroidism?
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The endocrine system consists of various glands that produce chemical messengers called hormones. Hormones help regulate your body’s functions. One of these glands is your thyroid, which sits at the base of the neck. Your thyroid secretes thyroid hormone, which regulates metabolism, development, and growth.

Sometimes, and not always for clear reasons, your thyroid starts producing too much or too little thyroid hormone. This results in an overt thyroid disorder, which typically includes symptoms and multiple abnormal thyroid biomarkers. For instance, people with hyperthyroidism overproduce thyroid hormones. And hypothyroidism is characterized by lower-than-normal thyroid hormone levels.

In some situations, only one of your thyroid test levels is outside the normal range, and symptoms may or may not be present. When this occurs, it is referred to as subclinical thyroid disease. It is possible to have subclinical hypothyroidism or hyperthyroidism.

In this article, we will focus on learning more about subclinical hyperthyroidism and why those with hypothyroidism (are at risk for developing it.

Detecting thyroid problems

Before jumping into subclinical hyperthyroidism, let’s review the main thyroid biomarkers:

  • Thyroid-stimulating hormone (TSH) is secreted from your pituitary gland in response to low thyroid hormone levels. It tells your thyroid gland to start making and releasing more thyroid hormone. When your thyroid functions well, the TSH level will fall within the reference range.
  • Thyroxine (T4) is the inactive form of thyroid hormone. Most thyroid hormones released from your thyroid will be in this form. For thyroid hormone to affect the body, cells must convert T4 to T3.
  • Triiodothyronine (T3) is the active form of thyroid hormone. Your thyroid gland secretes a small amount, but most T3 results from converting T4 into T3.

These three biomarkers work together to regulate your thyroid hormone secretion.

In response to low thyroid hormone levels, your pituitary gland secretes TSH. The TSH causes your thyroid gland to make and release thyroid hormones. Once your thyroid hormones reach a certain level, your pituitary gland stops secreting TSH. This drop in TSH signals your thyroid gland to stop releasing thyroid hormone. But, as more T4 converts to T3 and thyroid hormone levels drop, your pituitary gland senses the lower levels, again triggering the release of TSH. And the cycle continues. 

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Testing for a thyroid disorder

Why is measuring the TSH level the most common test for determining your thyroid function?

As illustrated above, thyroid hormone production depends on your thyroid gland receiving a signal from TSH. In thyroid disorders, TSH levels are either abnormally high or low, directly affecting thyroid hormone production.

Many healthcare providers don’t just order a TSH level. They order additional thyroid biomarkers tests such as the individual thyroid hormone levels (free thyroxine/free T4, and free triiodothyronine/free T3) and thyroid peroxidase antibodies (TPOAb). Measuring thyroid antibodies doesn’t evaluate your thyroid function. Instead, TPOAb levels can show if a thyroid autoimmune disorder is the cause of your thyroid condition.

Now with a basic understanding of thyroid hormone regulation and testing for thyroid disorders, let’s go back to subclinical hyperthyroidism.

What is subclinical hyperthyroidism?

Subclinical hyperthyroidism is characterized by a low TSH level with free T4 and T3 levels in the normal range. For comparison, those with overt hyperthyroidism have an overproduction of thyroid hormone. Typically, thyroid tests show a low TSH level with a high free T4 level.

Those with subclinical hyperthyroidism often present with no symptoms, and asymptomatic patients may not be aware of their thyroid condition. Others may have mildly adverse effects and nonspecific hyperthyroid symptoms, such as:

  • Diarrhea or loose stools
  • Fatigue
  • Fast heart rate, irregular heartbeat, changes in cardiac function, cardiovascular risk factors
  • Heart abnormalities, like palpitations or atrial fibrillation (Afib)
  • Anxiety or nervousness
  • Heat intolerance and increased sweating
  • Irregular menses
  • Muscle weakness
  • Unexplained weight loss
  • Increased appetite
  • Difficulty sleeping

Subclinical hyperthyroidism is further classified as either mild or severe based on the TSH level:

  • Mild subclinical hyperthyroidism: TSH level between 0.1 - 0.4 mIU/L
  • Severe subclinical hyperthyroidism: TSH less than 0.1 mIU/L

Another difference between the two subtypes is that mild subclinical disease may resolve independently, depending on the underlying cause. In contrast, those at the severe level have a high risk of progression to overt hyperthyroidism.

What causes subclinical hyperthyroidism?

The causes of subclinical hyperthyroidism are similar to those of overt hyperthyroidism. These causes include:

One cause of subclinical hyperthyroidism is excessive iodine ingestion. Iodine is an essential mineral that is needed for the production of thyroid hormones. However, consuming too much iodine can disrupt the balance of thyroid hormone production and lead to an overactive thyroid gland.

Another cause and risk factor for subclinical hyperthyroidism is excessive thyroid hormone replacement, also known as iatrogenic hyperthyroidism. Overmedication with thyroid hormones is a common cause of subclinical hyperthyroidism. 

Thyroid hormone replacement therapy and subclinical hyperthyroidism

An estimated 20% of those taking thyroid hormone replacement medication to manage hypothyroidism will experience subclinical hyperthyroidism. This situation usually results when the thyroid hormone replacement medication dose is too high.

Those whose thyroid hormone replacement includes a combination of T4 and T3 -- including natural desiccated thyroid hormone are at a higher risk.  Subclinical hyperthyroidism can occur with other thyroid hormone replacement medications but to a lesser extent.

It is essential to know that some over-the-counter thyroid hormone supplements that promote thyroid health contain thyroid hormones even if not listed on the label. If taken with thyroid hormone replacement medication, your thyroid hormones may increase, resulting in subclinical hyperthyroidism.

Treatment options for subclinical hyperthyroidism

The management of subclinical hyperthyroidism varies depending on the underlying cause, its severity, and the individual’s thyroid status.

As mentioned, some cases of mild subclinical hyperthyroidism will resolve on their own. Generally, these types of cases are related to pregnancy, postpartum, or a recent dose change in thyroid hormone replacement medication.

In these cases, your healthcare provider may monitor your symptoms and recheck your thyroid biomarkers in 3 to 6 months. If the underlying cause is medication related, adjusting your dose or switching to another thyroid hormone replacement medication may be warranted.

But, there are certain cases where your healthcare provider may recommend treating subclinical hyperthyroidism. Your healthcare provider may consider the following factors when determining if treatment is warranted:

  • Age
  • Risk factors for heart disease
  • Risk of fractures/bone loss/low bone mineral density/osteoporosis
  • Hyperthyroidism symptoms
  • Underlying cause

Treatment options in these cases may include antithyroid drugs such as methimazole (Tapazole), thyroid surgery, or radioactive iodine ablation (RAI). Beta-blockers may also be prescribed to help alleviate heart and blood pressure-related symptoms.

Treating subclinical hyperthyroidism can offer several benefits. First, it can help alleviate the symptoms associated with this condition, such as increased heart rate, anxiety, and weight loss. By addressing these symptoms, you can experience an improvement in overall well-being and quality of life. Additionally, treating subclinical hyperthyroidism can help prevent the progression of the condition into overt hyperthyroidism, which can have more severe health consequences. Early intervention and treatment can also help manage any underlying causes of subclinical hyperthyroidism, such as Graves’ disease or toxic nodular goiter. Overall, addressing subclinical hyperthyroidism can lead to better health outcomes and reduce the risk of complications in the long run.

A note from Paloma Health

Routine monitoring of your thyroid biomarkers can detect subclinical thyroid disorders or if your thyroid medication dose is too high. Early detection allows you to take steps to improve your thyroid health, including:

Routine monitoring can be a hassle, especially when you have to visit a lab or a doctor’s office to get blood drawn. Paloma Health’s thyroid testing kit measures the same thyroid biomarkers but from the convenience of your home. Our painless testing kit measures the three key thyroid biomarkers -- TSH, free T4, and free T3 -- as well as TPO antibodies. You also have the option to add on Vitamin D and reverse T3 (RT3) tests at checkout. 

Your results are reported in an easy-to-use patient portal with a personalized interpretation. You can print these results and share them with your healthcare provider. Or, take the next step and consult a knowledgeable Paloma thyroid practitioner, who can take an integrated approach to your hypothyroidism care. Get started with Paloma today!

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Sharma M, Aronow WS, Patel L, Gandhi K, Desai H. Hyperthyroidism. Medical Science Monitor. 2011;17(4):RA85-RA91. doi:

Donangelo I, Suh SY. Subclinical Hyperthyroidism: When to Consider Treatment. American Family Physician. 2017;95(11):710-716.

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Emilie White, PharmD

Clinical Pharmacist and Medical Blogger

Emilie White, PharmD is a clinical pharmacist with over a decade of providing direct patient care to those hospitalized. She received her Doctor of Pharmacy degree from Massachusetts College of Pharmacy and Health Sciences. After graduation, Emilie completed a postgraduate pharmacy residency at Bon Secours Memorial Regional Medical Center in Virginia. Her background includes caring for critical care, internal medicine, and surgical patients.

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