Introduction 

About hyperthyroidism and thyrotoxicosis

Hyperthyroidism is an illness from an extremely active thyroid gland, which leads to too much thyroid hormone in the blood.¹ These hormones set the speed of the body's energy use. Thyrotoxicosis is about the signs presented when there is too much thyroid hormone in the body. People often mix up hyperthyroidism and thyrotoxicosis, but they are not the same.

Why is it important to distinguish between the two?

It is important to tell these diseases apart as they need different care and drugs. Hyperthyroidism requires medication and treatments that help reduce thyroid production by the thyroid glands, whereas thyrotoxicosis requires treatment plans that target the symptoms that are presented, such as increased or irregular heart rate, tremors and anxiety. So, a care plan for hyperthyroidism won't fit thyrotoxicosis because the reasons for the symptoms between them are not the same. 

Overview of key differences and similarities 

In simpler terms, hyperthyroidism happens when glands work too much to produce thyroid hormones, while thyrotoxicosis is the effect, where symptoms arise from excessive thyroid hormones in the body. This article will explore the differences and similarities between hyperthyroidism and thyrotoxicosis.

Understanding hyperthyroidism 

What happens in hyperthyroidism?

The thyroid is a gland found at the front of the neck and consists of two lobes. It produces triiodothyronine (T3) and tetraiodothyronine (T4), commonly referred to as thyroid hormones. These are hormones that control the body's metabolism. The release of these hormones is governed by the hypothalamic-pituitary-thyroid axis. The decrease in T3 and T4 in the bloodstream sends a signal to the hypothalamus in the brain to release thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to release thyroid-stimulating hormone (TSH). Then, TSH signals the thyroid gland to produce T3 and T4 to compensate for the reduced thyroid hormones in the bloodstream. The TRH and TSH production are reduced once there are enough thyroid hormones in the body. 

In hyperthyroidism, the thyroid gland overproduces thyroid hormones, causing every organ in the body to malfunction. This can be due to either primary hyperthyroidism, where there are increased thyroid tissues that can produce excessive amounts of T3 and T4, or secondary hyperthyroidism, which is due to increased levels of TSH or TRH. Increased thyroid hormones affect the body’s metabolic rate and the nervous system, causing major issues in the body.

What can cause hyperthyroidism?

Graves’ disease 

Graves’ disease is an autoimmune disease where the body’s immune system causes the thyroid gland to produce excess thyroid hormones. The immune system produces autoantibodies that can stimulate the thyroid gland by attaching to the thyroid-stimulating hormone receptor (TSHR). Normally, the pituitary gland in the brain produces TSH, which regulates the release of thyroid hormone by attaching to the TSHR.³ Once there is an adequate amount of thyroid hormones in the bloodstream, TSH production reduces. In Graves’ disease, the autoantibodies will overstimulate the thyroid gland by continuously attaching to the TSHR causing continuous release of thyroid hormones to continue to be released into the bloodstream. In the UK, Graves’ disease is responsible for 70-80% of people with hyperthyroidism. 

Toxic multinodular or single nodular goitre 

Toxic multinodular or single nodular goitre presents as a swollen lump at the front of the neck. Multinodular goitre is the cause of hyperthyroidism in 15-20% of cases, and single nodular goitre accounts for 5% of the cases. These nodules are independently functioning tissues found in the thyroid gland, which continue to release thyroid hormones regardless of the thyroid hormones already present in the bloodstream. 

Toxic thyroid adenoma 

Toxic thyroid adenoma occurs when there is increased cell growth within the thyroid gland, called adenomas. Adenomas are caused by mutations within the cells, resulting in excess thyroid tissue that can produce thyroid hormones on top of the regular thyroid hormone produced by the thyroid glands, subsequently increasing the thyroid hormone levels in the body.⁴

Symptoms 

• Unintentional weight loss- People who have been diagnosed with hyperthyroidism tend to lose weight quickly because thyroid hormones tend to increase metabolism, causing the body to burn calories at a quick rate
• Tachycardia and palpitations- The thyroid hormones have a stimulating effect on the nervous system, which results in tachycardia or palpitations
• Heat intolerance- Hyperthyroidism often causes heat intolerance due to increased metabolism, which releases more heat in the body, causing the basal body temperature to increase
• Anxiety and irritability- Thyroid hormones can affect chemicals in the brain that are responsible for a person’s mood and mental well-being⁵

Testing for hyperthyroidism

Thyroid function tests (TFTs)

The National Institute for Health and Care Excellence (NICE) recommends thyroid function tests (TFTs) to diagnose people with thyroid imbalances,⁶ (shown in Table 1). This test is performed by a phlebotomist by taking a sample of blood to be tested for TSH, and Free T3 and T4 (thyroid hormones in the bloodstream). Doctors test TSH and T4 to check thyroid levels. Low TSH and high T4 mean you might have too much thyroid activity. T3 is key to knowing how bad it is.

Table 1. Thyroid function tests with their reference ranges; Free T3 and T4 refer to thyroid hormones found in the bloodstream.¹

Thyroid scan and ultrasound

Thyroid scans are done by experts to find harmful growths in the thyroid glands. A type of radioactive liquid is injected into the body that the thyroid glands soak up. Then, a small device checks for any harmful spots in the glands to make sure there are no goitres or adenomas.⁷

How to treat hyperthyroidism?

Antithyroid medications 

Taking medication by mouth is the best way to treat hyperthyroidism. Drugs, mainly carbimazole and propylthiouracil, help lower the thyroid levels in the body. These pills block the oxidation of iodine, which you need to make T3 and T4, which lowers the hormone levels in the blood.

Radioactive iodine therapy 

Radioactive iodine therapy is primarily used to destroy cancerous thyroid cells or abnormal growths. This can, however, damage all thyroid-producing cells within the glands, causing hypothyroidism, which is the lack of thyroid hormones in the body. To stop the side effects of the therapy, people get levothyroxine prescribed for life to regulate thyroid hormone levels.. Levothyroxine is usually taken to increase T3 and T4 hormones in the body. 

Surgery 

Surgical intervention is often required only in some cases, for example, in people with uncomfortably large goitres that interfere with their breathing and eating or in instances where thionamides do not work and radioactive therapy has been declined. This surgery involves either completely removing the thyroid glands, which is called a total thyroidectomy, or removing only part of the glands (hemithyroidectomy).

Understanding thyrotoxicosis 

What happens in thyrotoxicosis?

Unlike hyperthyroidism, thyrotoxicosis refers to the symptoms that result from excessive thyroid hormones in the body. These symptoms can be related to any organ in the body. There are many reasons for increased thyroid hormone levels in the blood, including hyperthyroidism.⁸

What can cause thyrotoxicosis?

There can be other causes of thyrotoxicosis besides hyperthyroidism.⁹ Some of these causes have been explored below.

Thyroiditis

Thyroiditis refers to the inflammation of the thyroid glands. Many reasons can result in thyroiditis, such as:

• Subacute thyroiditis- This condition is the inflammation of the thyroid glands due to a viral infection. People with this condition usually present with a painful and swollen thyroid, fever, tiredness and body aches. This condition can last up to 3-6 weeks¹⁰
• Postpartum thyroiditis- 1 in 20 women go through postpartum thyroiditis in the first year after childbirth, and do not present with any thyroid issues during their pregnancies. This occurs because the body’s immune system overreacts after being suppressed during the pregnancy¹¹
• Drug-induced thyroiditis- This condition occurs when drugs, such as anti-cancer, anti-viral and anti-arrhythmic drugs, are used. Some of these drugs can damage thyroid cells, causing thyrotoxicosis; therefore, people who are prescribed these medications must be monitored regularly

External thyroid hormone intake 

Thyrotoxicosis can be caused by ingestion of thyroid hormones, specifically levothyroxine. Levothyroxine is a medication used to treat hypothyroidism to increase free T3 and T4 levels. Occasionally, people can accidentally ingest more than the required dose, thereby causing thyrotoxicosis.

Malignant struma ovarii (Ectopic thyroid cancer)

Malignant struma ovarii (MSO) is a rare form of thyroid cancer (under 200 known cases) that is found in the ovaries. This causes more thyroid hormones because of extra hormone-making bits in the body.¹²

Symptoms

• Systemic symptoms (similar to symptoms seen in hyperthyroidism) 
• Cardiovascular effects (irregular heart rate and heart failure) 
• Neurological and psychological effects (tremors, anxiety) 

Diagnosis 

• Thyroid function tests- TFTs, as mentioned above, can help diagnose thyrotoxicosis as well. TFTs must be repeated 6 weeks after the first tests to confirm thyrotoxicosis. TFTs can vary in results based on the cause
• Diagnosing based on causes- Thyrotoxicosis can be diagnosed based on the causes, for example, in thyroiditis-induced thyrotoxicosis, inflammatory markers such as C-reactive protein and erythrocyte sedimentation rate (ESR) are elevated

Treatment approaches

Thyrotoxicosis is mostly treated according to the symptoms. This means the treatment fits the problems seen when someone is found with thyrotoxicosis. For instance, beta-blockers are used to calm an active nervous system. This can ease things like fast heartbeats or shakes. Treatment can also aim at the root causes of thyrotoxicosis, like hyperthyroidism, thyroiditis, or outside hormone use.

Key differences between hyperthyroidism and thyrotoxicosis 

The key differences between hyperthyroidism and thyrotoxicosis are mainly in the pathophysiology, causes, diagnosis and treatment plans, mentioned in Table 2.

Table 2. Key differences between hyperthyroidism and thyrotoxicosis

Key similarities between hyperthyroidism and thyrotoxicosis 

Hyperthyroidism and thyrotoxicosis are often used interchangeably because they have many similarities, including:

• Both involve excessive thyroid hormones in the body
• Overlapping clinical symptoms (weight loss, tachycardia, tremors, anxiety) 
• Common diagnostic tests (T3, T4, TSH levels) 
• Some shared treatment approaches (e.g., beta-blockers for symptom control) 

Summary

In summary, hyperthyroidism and thyrotoxicosis occur due to increased thyroid hormones. The key differences are, firstly, causation; hyperthyroidism is only caused by increased thyroid hormone secretion by the thyroid glands due to either primary or secondary hyperthyroidism. Secondly, TFTs can be used to diagnose hyperthyroidism when TSH levels are decreased and T3/T4 levels are increased, whereas thyrotoxicosis requires varying tests such as TFTs, thyroid scan to confirm Graves’ disease or multinodular goitres, and inflammatory markers for thyroiditis. Finally, the treatment can vary as well because hyperthyroidism treatment involves reducing thyroid hormone production by the thyroid glands, while thyrotoxicosis treatment requires symptomatic treatment, such as beta-blockers and steroids. It is important to diagnose both of these conditions accurately because they have different treatment plans that target distinct issues. 

FAQs

What causes thyrotoxicosis without hyperthyroidism?

Thyrotoxicosis can be caused by other means, such as inflammation of thyroid glands, ingestion of excess iodine or due to certain drugs.

Is hyperthyroidism also known as thyrotoxicosis?

Hyperthyroidism and thyrotoxicosis result in increased thyroid hormone levels. Additionally, hyperthyroidism can lead to thyrotoxicosis, but they are different conditions.

Does radioactive iodine therapy cause more cancer within the body?

Radioactive iodine can be carcinogenic to the body, but they are injected in very small amounts that are not harmful to the body.

What can happen if hyperthyroidism or thyrotoxicosis is left untreated?

If left untreated, these conditions can lead to a life-threatening condition called thyroid storm. This requires medical intervention to treat extreme thyrotoxicosis symptoms in multiple organs.

Who is more likely to be diagnosed with hyperthyroidism?

People who have a family history of hyperthyroidism (meaning someone within the family has hyperthyroidism) or other diseases like autoimmune diseases or deficiencies are likely to be diagnosed with hyperthyroidism. People who ingest high amounts of iodine or women who are postpartum are also likely to have hyperthyroidism.