Introduction

Your thyroid gland is an essential endocrine organ produces hormones.¹ These thyroid hormones are primarily responsible for regulating your metabolism, but they influence nearly every organ system in your body, including your brain, heart, muscles, and digestive system.¹ Under normal conditions, the level of these thyroid hormones in your circulation at any time is tightly controlled to maintain homeostasis (physiological balance within your body). However, when your thyroid becomes overactive, it can give rise to thyroid storm, also known as a thyrotoxic crisis.^1,2 

Thyroid storm is a potentially life-threatening condition characterised by inappropriately high levels of circulating thyroid hormones.² These excess hormones consequently overwhelm your body, impairing its ability to maintain normal function and leading to an array of complications, including tachycardia (abnormally rapid heart rate), hypertension (high blood pressure), confusion, and fever.³ To understand why this happens, we need to explore the pathophysiology (the biological mechanisms) behind a thyroid storm. 

Overview of Thyroid function and Thyroid hormone physiology

To understand the mechanisms behind the production of excess thyroid hormones during a thyroid storm, it is important to understand normal thyroid function and what normal thyroid hormone physiology looks like. 

Your thyroid gland produces and releases two primary types of hormones:

• Thyroxine (T4) – the most abundant thyroid hormone that is a precursor to its more biologically active form, T3.^1,4 T4 acts as a reserve hormone, as it is not very active until it has been converted to T3
• Triiodothyronine (T3) – this is the more active version that has direct effects on your body^1,4

These hormones play an important role in a number of functions, including ensuring normal growth and development, and regulating your metabolism, heart rate, and body temperature.² The production and release of these hormones are controlled by the combined efforts of your hypothalamus, pituitary gland, and thyroid gland. Together, these form a system called the hypothalamic-pituitary-thyroid (HPT) axis, which acts via negative feedback loops.^4,5 For example, when your body requires more thyroid hormones, the hypothalamus in your brain releases thyrotropin-releasing hormone (TRH), which stimulates your pituitary gland to release thyroid-stimulating hormone (TSH). TSH then signals to your thyroid gland to produce more T4 and T3 hormones.^4,5 Once sufficient hormone levels have been achieved, the negative feedback signals tell your brain and pituitary gland to stop releasing TRH and TSH, respectively. 

Pathophysiology of thyroid storm

The pathophysiology of thyroid storm is not yet fully understood,^1,2 however, it is known that it involves several mechanisms that contribute to excess circulating thyroid hormones, including:

Breakdown of the HPT axis regulation

In thyroid storm, dysregulation of the HPT axis can occur at multiple levels; however, it usually causes impairment of the negative feedback signals.^1,6 As thyroid hormone levels surge beyond normal physiological levels, your body fails to cope and instead enters a dangerously overwhelmed state.

Excess thyroid release

If your thyroid becomes overstimulated, it can lead to the sudden overproduction of T4 and T3 being released into your bloodstream. These effects are typically observed in certain thyroid conditions, such as Graves’ disease and Thyroiditis.^3,7 In Graves’ disease, an increase in the production and release of thyroid hormones occurs because your immune system makes antibodies that mimic TSH.³ This means your thyroid gland is constantly receiving signals to produce more hormones. In thyroiditis, thyroid tissue damage arising from inflammation causes stored hormones to leak uncontrollably into your bloodstream.³

Excess T4 conversion

During thyroid storm, the conversion of the reserve hormone, T4, is accelerated,⁶ meaning more of the active form, T3, enters your circulation and can exert its effects on multiple organs within your body. This happens because the enzyme responsible for converting T4 into T3 is upregulated, meaning there is more of it free to target T4 reserves.⁸ Factors such as infection and stress have been implicated in contributing to this enhanced conversion.

Decreased clearance of hormones

Under normal conditions, your liver and kidneys break down and remove excess thyroid hormones from your body. However, thyroid storm impairs their ability to do this, which allows the hormones to remain active and accumulate.^9,10

Systemic effects of excess thyroid hormones

Once a thyroid storm has begun, it creates a positive feedback loop, making it increasingly severe and more difficult to control without aggressive medical treatment. The excessive circulating thyroid hormones exert their effects on many organ systems within your body,^3,5 putting them under immense strain. Consequently, a number of symptoms arise.

Cardiovascular issues

Excess thyroid hormones put stress on your heart and increase your body’s sensitivity to adrenaline, which can cause tachycardia and arrhythmias (abnormal heartbeat). Over time, this strain on your heart can get increasingly worse, causing chest pain, hypertension, shock, and heart failure (in severe cases).^3,5

Neurological hyperactivity

Your brain and nervous system are directly affected by excess thyroid hormones and can become overstimulated. This can give rise to initial symptoms such as confusion or delirium, anxiety, and tremors, including muscle weakness. In worse cases, you may experience seizures and coma due to swelling in your brain.^3,5

Metabolic overdrive

Since thyroid hormones are responsible for regulating your metabolism, when their levels are out of control, it can cause hypermetabolism (when your metabolism goes into overdrive), leading to symptoms like fever, profuse sweating, increased oxygen demand, and energy depletion.^3,5 As a result, dehydration, electrolyte imbalance, and extreme exhaustion often occur.

Respiratory distress

The increase in your metabolic rate (specifically an increase in oxygen demand) during thyroid storm influences your breathing. You may find you have rapid and shallow breathing, with a feeling of breathlessness. In more severe cases, you may experience respiratory failure, as the muscles that are used for breathing weaken over time.^3,5

Digestive and gastrointestinal issues

Digestion is sped up by excess thyroid hormones, which can lead to nausea, vomiting, diarrhoea, and abdominal pain. In more severe cases, you may find your liver becomes overwhelmed and dysfunctional, which can cause jaundice (yellowing of the eyes and skin) and even liver failure.^3,5

Overactivation of immune responses

Sometimes your immune system gets involved by releasing proinflammatory molecules called cytokines. Cytokines drive inflammation, which exacerbates symptoms like fever, tissue damage, and, if severe enough, can lead to multiple organ failure.^3,5

Summary

In summary, a thyroid storm is a medical emergency that requires prompt diagnosis and often aggressive treatment. It is driven by excess thyroid hormone activity that creates a vicious cycle once initiated. Moreover, thyroid storm doesn’t affect just one part of your body, but rather affects almost every organ system, giving rise to a plethora of symptoms, from tachycardia and confusion to systemic inflammation and organ failure. Understanding the mechanisms that drive thyroid storm, such as the loss of regulation by the HPT axis, provides an explanation as to why this happens and how it can escalate so quickly.

 If you or somebody that you know has a thyroid condition, staying informed about the mechanisms, triggers, and early warning signs is important. This can help you prevent thyroid storm from occurring and know when to seek treatment before it becomes life-threatening.