What Is Subclinical Hyperthyroidism?

Introduction

Definition of subclinical hyperthyroidism

Subclinical hyperthyroidism is a medical condition characterised by lower-than-normal levels of thyroid-stimulating hormone (TSH) in the blood, alongside normal levels of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). Unlike overt hyperthyroidism, which presents with noticeable symptoms, subclinical hyperthyroidism often flies under the radar.¹

Importance of understanding subclinical hyperthyroidism

Understanding subclinical hyperthyroidism is of paramount importance due to the condition's latent yet potentially far-reaching impact on one's health. While patients may not manifest with obvious symptoms, this condition can serve as an early warning sign of thyroid dysfunction and other underlying health issues. It's worth noting that if left untreated, subclinical hyperthyroidism can progress to overt hyperthyroidism, resulting in more severe symptoms and complications. Furthermore, this condition can affect various bodily systems, such as the cardiovascular and skeletal systems, making it essential to address to prevent further health complications.

Understanding thyroid function

Overview of the thyroid gland 

The thyroid gland is a small, butterfly-shaped organ located in the front of the neck, just below the Adam's apple. Despite its modest size, it plays a pivotal role in regulating various bodily functions. The thyroid gland primarily produces two essential hormones: triiodothyronine (T3) and thyroxine (T4). These hormones are critical for controlling the body's metabolism, which includes processes such as energy production, heat regulation, and the functioning of various organs and systems. The thyroid gland is controlled by the pituitary gland, which secretes thyroid-stimulating hormone (TSH) to regulate the thyroid's hormone production. When stimulated by TSH, the thyroid releases T3 and T4 into the bloodstream, where they influence the metabolic rate and energy expenditure throughout the body. Understanding the basics of thyroid gland function is essential to grasp the intricacies of thyroid disorders like subclinical hyperthyroidism.²

Normal thyroid hormone production 

In a healthy individual, the production and release of T3 and T4 are finely balanced to maintain optimal bodily functions. When the body's metabolic rate decreases, or more thyroid hormones are needed, the pituitary gland releases TSH, which stimulates the thyroid gland to synthesise and release T3 and T4 into the bloodstream. Once in circulation, T3 and T4 exert their effects on various tissues and organs, ensuring that the body functions efficiently. The levels of these hormones are tightly regulated by a feedback mechanism involving the hypothalamus, pituitary gland, and thyroid gland. When thyroid hormone levels are sufficient, they signal the hypothalamus and pituitary gland to reduce TSH secretion, maintaining this delicate equilibrium. This normal thyroid hormone production is crucial for overall well-being, and any disruption, as seen in subclinical hyperthyroidism, can have significant health implications.³

Hyperthyroidism

Definition of hyperthyroidism 

Hyperthyroidism is a medical condition characterised by the overactivity of the thyroid gland, resulting in excessive production and release of thyroid hormones, primarily T3 and T4, into the bloodstream. This overproduction disrupts the body's normal metabolism and various physiological processes, causing the body to function at an accelerated pace. Individuals with hyperthyroidism typically have thyroid hormone levels that exceed the body's requirements, leading to a wide range of symptoms and potential health complications. These symptoms can affect multiple systems within the body, hence the importance of early detection and management.⁴

Key differences between subclinical and overt hyperthyroidism

Understanding subclinical hyperthyroidism requires differentiating it from overt hyperthyroidism. Overt hyperthyroidism is characterised by significantly elevated levels of thyroid hormones (T3 and T4) in the blood, accompanied by low or undetectable TSH levels. In contrast, subclinical hyperthyroidism exhibits subtle hormonal imbalances, with low TSH and normal T3 and T4 levels. The key difference lies in the severity of thyroid hormone imbalance and the presence of noticeable symptoms. Subclinical hyperthyroidism often lacks the overt signs of hyperactivity, making it a more challenging condition to diagnose without routine blood tests.

Causes of hyperthyroidism

Hyperthyroidism can have various underlying causes:⁵ 

• Graves' Disease: This autoimmune disorder can lead to overactivity of the thyroid gland, causing subclinical hyperthyroidism.
• Thyroid Nodules: Abnormal growths or nodules on the thyroid can autonomously produce thyroid hormones, disrupting the hormonal balance.
• Thyroiditis: Inflammation of the thyroid gland, whether due to infection or autoimmune processes, can temporarily release stored thyroid hormones into the bloodstream.
• Medications: Certain medications, such as amiodarone or excessive thyroid hormone replacement therapy, may contribute to subclinical hyperthyroidism.

Symptoms

The hallmark symptoms of hyperthyroidism encompass a range of physical and emotional changes. These may include unexplained weight loss, increased appetite, rapid heartbeat (tachycardia), sweating, heat intolerance, anxiety, tremors, irritability, fatigue, and muscle weakness. Additionally, some individuals may experience eye changes, known as Graves' ophthalmopathy, which can result in eye discomfort, bulging, and double vision. 

Risk factors

Risk factors for developing subclinical hyperthyroidism include: 

• Age: Older adults, especially women over the age of 60, are more susceptible to subclinical hyperthyroidism.⁶
• Gender: Women are at a higher risk of developing subclinical hyperthyroidism compared to men.⁶
• Family History: A family history of thyroid disorders, particularly autoimmune conditions, can predispose individuals to thyroid problems.⁷
• Medication History: The use of medications that affect thyroid function, such as amiodarone or lithium, may increase the risk.⁸
• Previous Thyroid Conditions: Individuals with a history of thyroid disorders or surgery may be more prone to developing subclinical hyperthyroidism.⁹
• Iodine Intake: Excessive iodine intake, through diet or supplements, can disrupt thyroid function and contribute to subclinical hyperthyroidism.¹⁰

Diagnosis and monitoring

Investigations for subclinical hyperthyroidism

Diagnosing subclinical hyperthyroidism typically involves a series of blood tests to measure the levels of key hormones. The diagnostic tests include:

• TSH Test: This is the primary screening test. In subclinical hyperthyroidism, TSH levels are typically low, indicating reduced pituitary gland stimulation of the thyroid.
• Free T3 and T4 Tests: These tests measure the levels of the thyroid hormones T3 and T4 in the blood. In subclinical hyperthyroidism, these hormone levels typically fall within the normal reference range.
• Thyroid Antibody Tests: If an autoimmune disorder like Graves' disease is suspected, tests for thyroid antibodies, such as thyrotropin receptor antibodies (TRAb), may be conducted to confirm the diagnosis.¹¹

Monitoring

The frequency of monitoring for subclinical hyperthyroidism may vary based on the individual's specific circumstances and the healthcare provider's recommendations. Individuals diagnosed with subclinical hyperthyroidism may be advised to undergo follow-up blood tests at intervals of 3 to 12 months. This regular monitoring helps to track changes in thyroid hormone and TSH levels,  providing valuable information about the stability of the condition. Adjustments to the monitoring schedule may occur depending on changes in the condition or the individual's health status.¹²

When to seek medical attention

It is important for individuals with subclinical hyperthyroidism to understand when to seek medical attention. Situations which should prompt medical review include: 

• Worsening symptoms: If they begin experiencing new or worsening symptoms, even if they are subtle, such as palpitations, unexplained weight loss, or mood changes.
• Pregnancy: For pregnant individuals, immediate medical attention is crucial, as subclinical hyperthyroidism can have implications for both the mother and the baby.
• Significant changes in thyroid hormone levels:  If follow-up tests reveal significant changes in TSH, T3, or T4 levels, this may indicate progression to overt hyperthyroidism.
• Thyroid-related complications: If complications related to thyroid function arise, such as cardiac arrhythmias or osteoporosis.
• Concerns about treatment: If there are concerns or questions about treatment options or the management of subclinical hyperthyroidism.¹³

Management 

There are various options for the treatment and management of subclinical hyperthyroidism.¹⁴

Monitoring

Situationally, especially when subclinical hyperthyroidism is mild and stable, healthcare providers may recommend close monitoring and observation without immediate intervention.¹²

Medication 

If the underlying cause is an autoimmune condition like Graves' disease, medications such as antithyroid drugs (e.g., methimazole or propylthiouracil) may be prescribed to suppress thyroid hormone production. 

Radioactive Iodine (RAI) Therapy

In certain cases, RAI therapy may be considered, particularly if pharmacological treatments cannot effectively manage the condition. RAI therapy aims to reduce thyroid hormone production.

Thyroidectomy 

Surgical removal of the thyroid gland (thyroidectomy) may be recommended for specific cases, such as when other treatments are ineffective or when there is a concern about thyroid nodules or thyroid cancer.

Lifestyle changes

• Diet: Limit iodine intake
• Stress Management: Use relaxation techniques
• Exercise: Adapt to energy levels
• Smoking Cessation: If applicable, quit smoking

Summary

Subclinical hyperthyroidism is a thyroid disorder in which there are low TSH levels and normal thyroid hormone levels. It often lacks noticeable symptoms and may be caused by autoimmune conditions, thyroid nodules, and the use of certain medications.  Risk factors include age and gender, and diagnosis is primarily based on blood tests.  Lifestyle changes can help manage the condition, and treatment options depend on the cause and severity. The prognosis varies, requiring ongoing medical supervision for long-term management. Understanding this condition is crucial for informed health decisions.

FAQs

What is subclinical hyperthyroidism?

It’s a condition where the thyroid gland produces excess thyroid hormones T3 and T4 but the levels of thyroid-stimulating hormone (TSH) - the hormone responsible for making the thyroid produce T3 and T4 - remain normal meaning this condition is hard to detect and often presents asymptomatically.

What causes subclinical hyperthyroidism?

Some common causes are Graves’ disease, excessive iodine intake, or side effects of medications.

How is subclinical hyperthyroidism diagnosed?

Diagnosis is done through blood tests that measure levels of TSH, T4, and T3.

What symptoms should I look for?

This condition often does not present with obvious symptoms however some subtle symptoms may be noticed such as rapid heart rate (tachycardia), mood changes, slight weight loss, and heat intolerance.

How is subclinical hyperthyroidism treated?

Treatment is dependent on the underlying cause however most commonly used treatments are administering medications that regulate thyroid hormone levels or radioactive iodine therapy. In some cases, removal of the thyroid may need to be done however this is usually if the condition has advanced.

How common is subclinical hyperthyroidism?

Prevalence of the condition increases with age and is based on risk factors such as gender (it’s most common in older women), comorbidities, and iodine intake.