Given their many shared symptoms and the fact that both are conditions that can be hard to diagnose, it may seem obvious that thyroid disease (specifically, hypothyroidism) and fibromyalgia could be connected to one another. Having one of these conditions may, in fact, predispose you to the other, and having both impacts each disease. Despite the associations, however, theories about the cause of these related conditions are not concrete. Researchers have proposed autoimmunity, dysfunction of the hypothalamus, viral and bacterial infections, and more possibilities.

Who They Affect

Both thyroid disease and fibromyalgia are very common in the United States.

Estimates suggest that at least 5 percent of adults in the United States are living with hypothyroidism, though this number does not include the roughly 13 million people thought to be undiagnosed. (Autoimmune hypothyroidism or Hashimoto’s disease is the most common form.)

According to the Centers for Disease Control and Prevention (CDC), 2 percent of the population, or around 4 million people, are living with fibromyalgia.

Combined Occurrence

Not only are hypothyroidism and fibromyalgia both common, but they frequently occur together. According to a 2017 study, the prevalence of fibromyalgia in the general population was 2 percent to 7 percent, but as high as 30 percent to 40 percent in people with hypothyroidism. Using newer diagnostic criteria for fibromyalgia, the incidence rose to 62 percent.

Among people with hypothyroidism, fibromyalgia was more common in those who were overweight, had positive anti-thyroperoxidase antibodies, and who had been living with thyroid disease for a long period of time.

Similarities in Epidemiology

Thyroid disease and fibromyalgia tend to affect a similar population of people:

• Sex: Both conditions are much more common in women.
• Age: Both conditions are most common during the childbearing years and in middle age (though they can occur at any age).

Similarities such as these have played into some hypotheses about potential causes.

Potential Causes

Hashimoto’s disease is an autoimmune disease in which autoantibodies (antibodies against self) damage thyroid tissue. While that is confirmed, there is a great deal of controversy over the underlying cause of fibromyalgia.

Some have postulated that both autoimmune thyroid disease and fibromyalgia are caused by a similar underlying process that can predispose a person to both conditions, but neither this nor any other theory has been confirmed.

Some of the possibilities that have been suggested include:

Autoimmunity

The autoimmune basis of Hashimoto’s disease can be demonstrated with the presence of anti-thyroperoxidase (TPO) antibodies. A 2017 study found a potential link to autoimmune disease with fibromyalgia as well. Compared to the general population, people with fibromyalgia had a high degree of positivity to TPO antibodies. Theories vary, though some have postulated that underlying autoimmune disease precipitates the development of bothconditions.

While some researchers believe that fibromyalgia may also be an autoimmune disease, others think that symptoms thought to point to fibromyalgia may actually be symptoms related to hypothyroidism.

Suppression of the Hypothalamus

The hypothalamus is a region of the brain that has been likened to the utility center. It functions to maintain homeostasis, or a steady state in the body by regulating hormone levels (including thyroid hormones), sleep, temperature, hunger, and the autonomic nervous system (the involuntary part of the nervous system that controls blood pressure, blood flow, digestion, and much more).

Suppression of the hypothalamus may occur due to problems with the mitochondria in hypothalamic cells, the organelles that are considered the “energy furnaces” of cells.

Some researchers have suggested that such dysfunction of the hypothalamus may be the root cause of both disorders and that, by itself, hypothalamic dysfunction can cause many of the symptoms attributed to thyroid disease and fibromyalgia.

Viral or Bacterial Infections

Both autoimmune disease and fibromyalgia sometimes occur following an infection with the Epstein-Barr virus (the cause of mononucleosis) or Lyme disease.

Epstein-Barr virus infections, in particular, have been implicated in autoimmune diseases when a component of the virus resembles proteins in the body (so that antibodies are “accidentally” produced against self) or due to the immune system being overactivated in response to the infection.

Trauma

As with some infections, both autoimmune thyroid disease and fibromyalgia are sometimes preceded by trauma. Trauma, in turn, lends credence to an autoimmune basis, as it activates the immune system. This activation might lead to a hyperactive state (causing autoimmunity) after the injury has resolved.

Hormonal Problems

Other hormonal problems, such as low cortisol production by the adrenal gland, have also been postulated as underlying causes of both conditions.

Shared Symptoms

There are a number of symptoms that are similar between hypothyroidism and fibromyalgia as well as some that are more common with one condition than the other.

Symptoms that are very common among both people with hypothyroidism and those with fibromyalgia include:

• Fatigue, exhaustion, and non-restful sleep
• Body aches (muscle and joint pain)
• Cognitive problems often referred to as “brain fog”
• Depression and anxiety

Symptoms More Common in Hypothyroidism

• Weight gain
• Thinning of hair
• Constipation
• Dry skin
• Menstrual irregularities

Symptoms More Common in Fibromyalgia

• Headaches
• Paresthesias (abnormal sensations such as tingling of the extremities)
• Muscle spasms
• Cognitive concerns

Autoimmune thyroid disease often leads to a worsening of fibromyalgia symptoms. Fibromyalgia may also enhance the symptoms of thyroid disease, or make it more difficult to know if thyroid hormone replacement therapy is optimal.

Diagnosis

Hypothyroidism and fibromyalgia have an unfortunate commonality in that they are both frequently misdiagnosed.

Hypothyroidism

The diagnosis of hypothyroidism often begins with a TSH test, though some experts believe that a normal TSH is insufficient to rule out thyroid disease, especially in those who have a family history of an autoimmune thyroid condition (such as Hashimoto’s thyroiditis or Graves’ disease). Additional diagnostic tests may be ordered to paint a fuller picture of your thyroid health, including total and free T4, total and free T3, and thyroid antibody tests.

Note, too, that people with thyroid disease who have thyroid autoantibodies are also more likely to have fibromyalgia.How to Understand Thyroid Function Tests and Normal Ranges

Thyroid testing is extremely important as hypothyroidism is not only underdiagnosed, but frequently misdiagnosed as premenstrual syndrome, depression (including postpartum depression), stress, and other conditions.

Fibromyalgia

The diagnosis of fibromyalgia is primarily a clinical diagnosis based on symptoms of pain, fatigue, and cognitive symptoms. Since under-treated hypothyroidism can cause symptoms that are very similar, it can be challenging to know whether a person meets the criteria for fibromyalgia with hypothyroidism.

Before diagnosing fibromyalgia, thyroid function testing should be done. If autoimmune hypothyroidism is found, it’s reasonable to treat the thyroid condition alone at first and see if pain resolves. If a person is already on thyroid replacement therapy and has symptoms of fibromyalgia, the goal should be optimal control before a diagnosis is made.

Differential Diagnoses

To further complicate matters, there are other conditions that may cause symptoms that are similar to fibromyalgia and hypothyroidism. These other possible diagnoses can make the two conditions less obvious and challenging to identify, perhaps even masking the development of fibromyalgia with hypothyroidism and vice versa.

Other diagnoses that may also be considered include:

• Anemia
• Depression and/or anxiety
• Vitamin D deficiency
• Kidney disease
• Liver disease
• Infections (such as mononucleosis, Lyme disease, and more)
• Sleep apnea
• Hormonal conditions (such as menopause or premenstrual syndrome)
• Other autoimmune diseases (such as lupus)

Living With Either Condition

The connection between hypothyroidism and fibromyalgia is more than academic, and an awareness of this overlap may be important whether you have one of these conditions alone or a combination of both.

Tips for People with Hypothyroidism

Before diagnosing fibromyalgia in people with hypothyroidism, some physicians recommend checking thyroid tests in addition to TSH. In a theory referred to as “cellular hypothyroidism,” these researchers claim that having a normal TSH doesn’t necessarily mean that a person has an adequate amount of T3 circulating in their bloodstream.

In the body, T4 is converted to T3, which is the active form of the hormone at a cellular level. The theory is that a lack of T3 (due to an impairment in the conversion of T4 to T3 in the body) may be insufficient to maintain normal metabolism in cells. We do know that some signs of hypothyroidism, such as an elevated cholesterol level, may persist despite a normal TSH (but become lower when TSH is suppressed), giving indirect support to this theory.

Proponents of this theory claim that levothyroxine alone is not enough to raise circulating T3 levels and treatments such as combination T4/T3 therapy or natural desiccated thyroid therapy should be considered.

Tips for People With Fibromyalgia

Even if you have had normal thyroid tests in the past, if your fibromyalgia is worsening or not responding to treatment, ask your doctor to repeat your thyroid tests.

Thyroid disease and fibromyalgia have many commonalities, though the exact connection between the two conditions is still unclear. What is better understood, however, is that it’s important to be aware of both conditions and potential interactions when making a diagnosis and selecting treatment options.

Credit: verywellhealth