Introduction

Have you ever wondered how your body keeps your energy levels balanced, or what happens when that balance becomes unmanageable? 

The thyroid is at the centre of this balancing act, a small butterfly-shaped gland in the neck that produces thyroid hormones.¹ Sometimes, for various reasons, the thyroid can become overactive, leading to a condition known as thyrotoxicosis. This condition can lead to uncomfortable symptoms and potentially dangerous health complications.² One of the most noticeable signs of thyrotoxicosis is unexplained weight loss, which can take a serious toll on someone who has the condition. 

This article will go into what thyrotoxicosis is, how the condition causes weight loss, and how we can diagnose and treat the condition.

What is thyrotoxicosis?

Thyrotoxicosis happens when the thyroid gland produces too much thyroid hormone in the body. Thyroid hormones are responsible for controlling metabolism, which is the process the body goes through to turn the food we eat into energy for things like:^2,3

• Body temperature
• How fast does the heart beat
• Attention span
• Mood and sleep

The amount of thyroid hormones required for the body varies depending on factors like exercise and activity levels, age, and even temperature. When someone has thyrotoxicosis, their metabolism becomes too fast, leading to symptoms that can have effects on the entire body, including:

• Increased sweating
• A fast or irregular heartbeat (tachycardia)
• Shakiness
• Needing to go to the toilet more often
• Sensitivity to heat 

Thyrotoxicosis can vary in severity from mild to severe. The symptoms can be more intense depending on the severity of thyrotoxicosis. Severe cases of thyrotoxicosis are called a thyroid storm, and the symptoms are even potentially life-threatening. Individuals suffering from thyroid storm might have a high fever, be disoriented or confused, or may even lose consciousness.⁶

Causes of thyrotoxicosis

Thyrotoxicosis can be caused by a variety of reasons, including:⁴

• Graves’ disease is an immune condition where the immune system makes antibodies which activate the thyroid, making it produce too much thyroid hormone
• Lumps which are growing on the thyroid (known as nodules) can produce extra thyroid hormones independent of the thyroid itself 
• Taking medications such as amiodarone, which are used to treat irregular heartbeats (arrhythmias), can sometimes cause thyrotoxicosis 
• In rare cases, thyrotoxicosis can be caused by adenomas, a type of non-cancerous tumour which can produce more thyroid hormone along with the normal gland

According to the NHS, Graves' disease is the most common cause of thyrotoxicosis, with roughly 4 in 5 individuals with an overactive thyroid also being diagnosed with Graves’ disease. Other causes, such as thyroid nodules and medication-related thyrotoxicosis, are less common. 

There have been some reported cases of individuals taking thyroid hormone medications, which are normally taken by individuals with an underactive thyroid, usually to lose weight. These individuals are known as having factitious hyperthyroidism and may experience symptoms closely resembling thyrotoxicosis. Misuse of thyroid medication in this way is extremely dangerous and has been shown to lead to serious complications, including long-term heart conditions, if the medication is taken for an extended period of time.⁵

Diagnosis of thyrotoxicosis

Doctors usually diagnose individuals with thyrotoxicosis after they report noticeable symptoms such as sudden and unexplained weight loss, trouble sleeping (insomnia), or an unusually fast heartbeat. To confirm the diagnosis, doctors may also arrange for blood tests known as thyroid function tests (TFT).² Here, they check the levels of:

• Thyroid-stimulating hormone (TSH) – this is a hormone produced by another gland in the brain known as the pituitary, and regulates many of the processes in our body, including the amount of hormones the thyroid produces
• Triiodothyronine (T3) and tetraiodothyronine (T4) – these are two of the main thyroid hormones, and are responsible for controlling the body’s metabolism

In most cases, doctors will look at TSH levels first, before checking T3 and T4 levels to confirm the diagnosis. If the test results show that an individual has low TSH levels alongside increased values of T3 and/or T4, they are very likely to have thyrotoxicosis.

Why does thyrotoxicosis cause weight loss?

The thyroid gland is a major factor in controlling the body’s metabolism through the thyroid hormones. Studies suggest that people with thyrotoxicosis have an increased metabolism rate of up to 50%, meaning the body burns energy much more quickly.¹ This means the body constantly uses calories from food, even when the individual is resting or asleep. Because of this faster metabolism, many individuals with thyrotoxicosis have substantial weight loss, even if they eat more food or have an increased appetite.^8,9

As a result of faster metabolism, the body runs out of energy that it gets from food and starts to break down tissues in the body, like muscles and fat, for energy. This can make weight loss worse and even lead to muscle weakness in moderate to severe cases of thyrotoxicosis. 

Studies have also shown that thyrotoxicosis can speed up the digestion and movement of food through the stomach and intestines, which can make it even more difficult for the body to get energy from food.⁷ These factors all contribute to the quick and unexplained weight loss associated with most cases of thyrotoxicosis.

Treatment and managing weight loss

Thyrotoxicosis is easily treatable, and the recommended treatment depends on the underlying cause of the condition. Symptoms like weight loss resolve once the thyroid hormone levels return to normal. Treatment methods include:^2,9

• Medication such as methimazole is used to reduce the amount of thyroid hormone made
• Radioiodine therapy, a specialised type of radiotherapy which aims to destroy thyroid gland cells to reduce the amount of thyroid hormones being produced
• In more severe cases, surgery, to remove part or all of the thyroid gland (thyroidectomy), may be required to stop the gland from producing too much thyroid hormone

Usually, treatment options will depend on how severe the thyrotoxicosis is and other factors like age, overall health, and whether the individual has any other preexisting conditions or diseases. Typically, a doctor specialising in hormonal conditions (endocrinologist) will discuss the best course of treatment with the individual.

Conclusion

Whilst many people might see weight loss as an upside of thyrotoxicosis, it could be a telltale sign that something has gone awry in the body. Whilst weight loss could be an unexpectedly pleasant side effect at first, it is also often a signal that the thyroid is overactive, causing changes in the entire body that can be dangerous or even life-threatening if left untreated. 

By recognising these symptoms quickly and seeking medical advice as soon as possible, thyrotoxicosis can be easily managed and treated, preventing any further health complications that could arise if the condition is left untreated.

Summary

The thyroid gland, a hormone-releasing organ in the neck, is responsible for keeping the body’s metabolism balanced. Sometimes, the thyroid can become overactive and produce too much thyroid hormone, leading to a condition called thyrotoxicosis. This increase in thyroid hormone production speeds up the body’s metabolism by up to 50%, which can lead to unpleasant effects such as trouble sleeping, shakiness, and unexplained weight loss. 

Thyrotoxicosis can arise from a wide range of causes. One of the most common causes is Graves’ disease, a condition where the immune system triggers the thyroid to produce too much thyroid hormone. Other causes can include side effects from medications or, in rarer cases, benign tumours on the thyroid gland. A small number of people may also take thyroid hormone medications, which are prescribed to treat an underactive thyroid, as a method of losing weight easily. Misuse of medication is extremely dangerous and can also lead to serious health complications and symptoms almost identical to thyrotoxicosis.

Medical specialists usually diagnose thyrotoxicosis using a combination of recognising symptoms and blood testing. The condition is very easily treatable if found early, with treatment options ranging from medication to more permanent solutions such as surgery to remove the thyroid gland.