Hyperthyroidism vs. Hypothyroidism: Key Differences Explained

The thyroid is a small but mighty endocrine gland that plays a big role in regulating metabolism and energy. According to the American Thyroid Association (ATA), an estimated 20 million Americans suffer from some form of thyroid disease, but up to 60% don't know it.

Hypothyroidism and hyperthyroidism are two of the most common thyroid disorders. Although their names may sound the same, these two conditions are very different from one another.

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Understanding the Thyroid Gland

The thyroid gland, located at the base of the neck, is a butterfly-shaped organ that plays a central role in regulating the body's metabolism. It achieves this by producing two hormones: triiodothyronine (T3) and thyroxine (T4). These hormones control how the body converts food into energy, influencing heart rate, body temperature, and overall metabolic function. 

The thyroid is controlled by the pituitary gland, which releases thyroid-stimulating hormone (TSH) to signal the thyroid to release more or less T3 and T4, depending on the body's needs.

When the thyroid functions properly, it maintains a delicate balance of hormone production. However, when this balance is disrupted, it can result in either hyperthyroidism or hypothyroidism. Both conditions can significantly impact health and quality of life.

What Is Hyperthyroidism?

Hyperthyroidism describes an overactive thyroid gland and excessive amounts of thyroid hormones in circulation.

Causes of Hyperthyroidism

Hyperthyroidism can result from several underlying conditions, with the most common being Graves' disease, an autoimmune disorder that stimulates the thyroid gland to produce excess hormones. 

Other causes include thyroid nodules (thyroid adenoma or toxic nodular goiter), inflammation of the thyroid (thyroiditis), and excessive iodine intake. 

Symptoms of Hyperthyroidism

Individuals with hyperthyroidism may experience a variety of symptoms reflecting an increased metabolism, including: 

• Increased heart rate (tachycardia)
• Unexplained weight loss
• Increased appetite
• Anxiety and irritability
• Heat intolerance and excessive sweating
• Diarrhea
• Tremors

Diagnosis and Treatment

Diagnosing hyperthyroidism involves a combination of a detailed medical history, physical examination, and laboratory tests.

Physical Exam Findings

• Enlargement, tenderness, or nodularity of the thyroid gland
• Swelling, redness, and bulging of the eyes
• Rapid and/or irregular heartbeat
• Tremor in the hands
• Warm, moist, and thinning skin (26) 

Blood Tests

The most common laboratory results indicative of hyperthyroidism include low TSH with high levels of T3, T4, and thyroid-stimulating immunoglobulin (TSI).

Imaging Studies

• A thyroid ultrasound uses high-frequency sound waves to create an image of your thyroid. It can visualize thyroid enlargement, inflammation, and nodules. (3) 
• A radioactive iodine uptake (RAIU) test measures the amount of radioactive iodine the thyroid absorbs to assess thyroid function. High RAIU is measured in Graves' disease, toxic adenoma, and toxic multinodular goiter. (30) 

Treatment options vary depending on the underlying cause but focus on bringing thyroid hormones back into a normal range and alleviating hyperthyroid symptoms. The most common treatment options include antithyroid and beta-blocker medications, radioactive iodine therapy, or thyroidectomy (surgical removal of the thyroid). (30) 

What Is Hypothyroidism?

Hypothyroidism is when the thyroid gland is underactive, leading to insufficient thyroid hormone production.

Causes of Hypothyroidism

Hypothyroidism is most commonly caused by Hashimoto's thyroiditis, an autoimmune disorder in which the immune system attacks the thyroid gland, leading to decreased hormone production. 

Other causes include congenital defects, surgical removal of the thyroid, radiation therapy, thyroiditis, certain medications (such as lithium), iodine deficiency, and pituitary gland dysfunction (57). 

Symptoms of Hypothyroidism

Symptoms of hypothyroidism are reflective of a slowed metabolism and include:

• Fatigue
• Brain fog
• Slow heart rate (bradycardia)
• Weight gain
• Sensitivity to cold
• Dry skin
• Constipation
• Depression

Diagnosis and Treatment

Diagnosis of hypothyroidism is typically made through a combination of a comprehensive medical history, physical examination, and laboratory tests.

Physical Exam Findings

• Enlargement, tenderness, or nodularity of the thyroid gland
• Slow and/or irregular heartbeat
• Cold skin
• Loss of the outer one-third of the eyebrows
• Delayed deep tendon reflexes (61) 

Blood Tests

Laboratory findings diagnostic for hypothyroidism include elevated TSH with low levels of T3 and T4. Elevations in thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb) suggest Hashimoto's thyroiditis. 

Imaging Studies

• A thyroid ultrasound often shows an enlarged thyroid gland with an uneven texture and decreased blood flow (2). 
• Hypothyroidism causes decreased iodine uptake during an RAIU test (28).

The primary treatment for hypothyroidism is hormone replacement therapy, most commonly with synthetic levothyroxine (T4), which restores hormone levels and alleviates symptoms (61). Sometimes, your doctor may prescribe synthetic liothyronine (T3) or desiccated thyroid extract (combined T4 and T3 derived from pig thyroids).

Key Differences Between Hyperthyroidism and Hypothyroidism

The main difference between hyperthyroidism and hypothyroidism is the amount of thyroid hormone produced by the thyroid gland, which directly affects metabolism and symptomology.

Metabolic Rate and Energy Levels

Thyroid hormones regulate the body's basal metabolic rate (BMR), which is the amount of energy expended while at rest. T3 binds to cell receptors to stimulate processes that control energy production and expenditure. It increases mitochondrial activity, leading to greater production of ATP (adenosine triphosphate), the energy currency of the cell. This results in elevated oxygen consumption and heat production. (41) 

When thyroid hormone levels are low, BMR decreases, leading to reduced energy expenditure and symptoms like weight gain, bradycardia, fatigue, and cold intolerance (39, 41). 

Conversely, excessive thyroid hormone levels increase BMR in hyperthyroidism, causing higher energy expenditure and more heat production. This can lead to weight loss, tachycardia, heat intolerance, increased appetite, and even muscle wasting. (39, 41) 

Mood and Mental Health

Cognitive and mental illnesses have been linked to clinical thyroid disease. 

Hypothyroidism frequently manifests as cognitive impairment and sadness (36). One meta-analysis found that people with hypothyroidism were over three times more likely to score higher on standardized depression assessments compared to those without. 

Hyperthyroidism, on the other hand, more commonly presents with agitation, anxiety, and psychosis (36). According to one study evaluating the impact of hyperthyroidism on mental health, 40% of patients had moderate-to-severe anxiety.

Shared Symptoms and Overlapping Conditions

Although hyperthyroidism and hypothyroidism are characterized by opposite effects on the body's metabolism, they can share some overlapping symptoms. For instance, both conditions can cause: 

• Fatigue 
• Mental health issues 
• Menstrual irregularities 
• Fertility issues 
• Muscle pain and weakness 
• Sleep disturbances 
• Hair loss 
• Tinnitus (ringing in the ears) 
• Goiter (an enlarged thyroid gland), which can cause difficulty swallowing and breathing 

Thyroiditis, inflammation of the thyroid gland caused for various reasons, often follows a distinct three-phase course that can include both hyperthyroid and hypothyroid stages. 

• Thyrotoxic Phase: Inflammation causes the release of stored thyroid hormones into the bloodstream, resulting in symptoms similar to hyperthyroidism.
• Hypothyroid Phase: Inflammation progresses, and the thyroid gland becomes less capable of producing sufficient hormones.
• Recovery Phase: Most experience a return to normal thyroid function, but some may develop chronic thyroid issues.

Long-Term Complications if Left Untreated

It's important to correctly diagnose thyroid conditions, as both hyperthyroidism and hypothyroidism can cause serious health complications when left untreated. 

Complications of Hyperthyroidism

• Cardiovascular Problems: Increased risk of heart disease, including atrial fibrillation, high blood pressure, and heart failure due to elevated heart rate and strain on the heart (19). 
• Bone Health: Higher likelihood of osteoporosis and fractures due to accelerated bone loss caused by excess thyroid hormones (40). 
• Thyroid Storm: A rare but life-threatening condition characterized by an extreme exacerbation of hyperthyroid symptoms, requiring immediate medical attention (48). 

Complications of Hypothyroidism

• Cardiovascular Problems: Increased risk of heart disease due to elevated cholesterol levels and hypertension (high blood pressure) resulting from slowed metabolism (52). 
• Obesity: Greater likelihood of obesity due to decreased metabolic rate and weight gain (43). 
• Myxedema Coma: A rare but serious condition characterized by extreme hypothyroidism that can lead to unconsciousness and requires immediate medical intervention (54). 

When to See a Doctor

Schedule an appointment to discuss thyroid health if you have a new onset of symptoms, including unexplained changes to weight, digestion, heart rate, body temperature, sleep patterns, energy levels, or mood. 

Patients with preexisting thyroid disease should also consult a doctor if they begin to experience thyroid symptoms, as this indicates treatment is no longer working and needs to be modified. 

Living With Thyroid Disorders

Regardless of whether you have hyperthyroidism or hypothyroidism, an integrative treatment plan to support thyroid health and function can include (57, 59):  

• Eating a whole-food, anti-inflammatory diet that is rich in thyroid-supportive nutrients and supports gut health. 
• Manage stress by getting adequate amounts of sleep, fostering social relationships, exercising regularly, practicing mind-body therapies, and taking adaptogenic supplements like ashwagandha. 
• Limit exposure to endocrine-disrupting chemicals (EDCs) by reducing the use of plastic products, opting for glass or stainless steel containers for food storage, avoiding personal care products with parabens or synthetic fragrances, and buying organic produce.

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Key Takeaways

• Hypothyroidism and hyperthyroidism are distinct thyroid disorders characterized by opposite thyroid hormone levels. Hypothyroidism is caused by thyroid under-activity, while hyperthyroidism results from thyroid overactivity.
• These conditions have opposite effects on metabolism. Hypothyroidism slows metabolism, leading to symptoms like weight gain, fatigue, and cold intolerance. Hyperthyroidism speeds up metabolism, causing weight loss, anxiety, and heat intolerance.
• While both conditions require medical intervention to restore normal thyroid hormone levels, treatment options will vary depending on whether the goal is to suppress excessive thyroid hormone production or supplement low thyroid hormone levels.
• Patients should be mindful of new symptoms, such as unexplained changes in weight, energy, or mood, which may signal thyroid dysfunction. With proper diagnosis and treatment, both hypothyroidism and hyperthyroidism are manageable.