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Despite its tiny size, the thyroid gland plays a major role in regulating body metabolism and growth. In addition to controlling the pace of metabolism, this gland is also involved in regulating body temperature.
As well as these effects on the physical body, thyroid function is also related to mood regulation. Someone with a dysfunctional thyroid gland can potentially be affected by any of a number of mood disorders. These tend to affect women disproportionately because women are more prone to thyroid disorders than men.
In people with some form of mood disorder, thyroid dysfunction is much more common than it is in the general population, indicating a clear and strong link between thyroid and mood disorders. People with mood disorders have a 20% to 30% increased risk of having a thyroid disorder (Holsboer). It is, however, not always clear whether thyroid disorder is a consequence of the mental illness, or whether the thyroid disorder is a predisposing factor or cause.
In general there are two types of thyroid dysfunction. Disorders in which the thyroid gland is underactive are called hypothyroid disorders. Disorders in which the thyroid is overactive are hyperthyroid disorders.
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Hyperthyroidism and Mood Disorders
In hyperthyroidism, the thyroid gland produces too much thyroid hormone. This can have a number of causes, with one common one being a condition called Grave's disease. This is where the thyroid gland becomes enlarged, leading to an overproduction of thyroid hormone. Another common cause of hyperthyroidism is a benign or malignant tumor.
When too much thyroid hormone is present, the body goes into overdrive. The body's metabolism increases significantly, leading to symptoms such as excess weight loss, sweating, heat sensitivity, rapid heartbeat, heart palpitations, and diarrhea.
In addition, mood and mental health can become affected. People with hyperthyroidism are often affected by a symptom called dysphoria, which might take the form of anxiety, irritability, depression, or restlessness of mood.
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Mood Disorders Related to Hypothyroidism
Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormone. Common causes of hypothyroidism include iodine deficiency, a deficiency of thyroid-stimulating hormone, and an autoimmune disease called Hashimoto’s thyroiditis, which causes destruction of thyroid cells. Hypothyroidism can also be caused by certain types of mood-stabilizing medications, in particular lithium-based drugs used to treat bipolar disorder.
A person with an underactive thyroid gland is likely to have symptoms such as excess weight gain, muscle cramps, joint pain, fatigue, constipation, brittle hair and fingernails, dry skin, cold sensitivity, and low heart rate.
Mood disorders such as depression, mood swings, and irritability are also strongly associated with hypothyroidism, as are sleep disturbances.
People with mood disorders often have a condition called subclinical hypothyroidism, in which thyroid hormone levels appear normal in blood test results, but are in fact abnormal. This subclinical hypothyroidism, which is thought to be the result of autoimmune disease, has been shown to be a significant risk factor for depression.
In addition, people with bipolar disorder are more likely to have subclinical hypothyroidism than people with so-called unipolar depression. People with bipolar disorder who rapidly cycle between mood states are highly likely to have hypothyroidism. This is the case even in people whose hypothyroidism is not caused by the mood-stabilizing medications they are taking.
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Thyroid Function Treatments to Improve Mood Disorder Symptoms
Where a mood disorder is found to be caused by or related to abnormal thyroid function, treatment can include therapy with one or more synthetic thyroid hormones. This combination of treatments is highly desirable due to the strong association between thyroid hormones and mood disorders. When a mood disorder is a symptom of overactive or underactive thyroid, treatment for the thyroid condition itself can effectively treat the mood disorder.
People with hypothyroidism are generally treated with one or more synthetic thyroid hormones, depending on the specific hormones they are deficient in. For someone with hyperthyroidism, the treatment is very different, as there is no medication that can directly reduce the amount of hormones the thyroid produces. Therefore, hyperthyroidism is often treated with surgery that removes the gland entirely.
Someone who is treated for hypothyroidism or hyperthyroidism will usually find that it takes several months for his or her mood disorder symptoms to start improving.
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Combination Therapy: Mood Stabilizers, Antidepressants, and Thyroid Hormones
Sometimes, thyroid dysfunction is not the direct cause of a mood disorder, but is still contributing to the development of the disorder and its symptoms. In these cases, the thyroid dysfunction is usually a type of hypothyroidism, and treatment might involve a combination of different medications, such as thyroid hormones and antidepressants, or other types of mood-stabilizing drugs.
An important consideration when it comes to treatment of thyroid and mood disorders is that thyroid hormones can interact with some antidepressants in unexpected ways. In some people, for example, certain antidepressant classes can reduce the efficacy of thyroid hormone.
Another piece of evidence regarding this interaction is a study in which people being treated with antidepressants for panic disorder actually had a better response to the drugs when they also received thyroid hormone treatment (Holsboer). One reason for this might be that antidepressants somehow interfere with interactions between brain cells and thyroid hormones, making the hormones less effective.
For people with bipolar disorder, an important facet of medication therapy is treating clinical or subclinical hypothyroidism. This is particularly important because, as noted previously, certain mood-stabilizing medications can cause hypothyroidism. In addition, multiple studies have shown that people with bipolar disorder respond much more positively to mood stabilizers when they also receive treatment for hypothyroidism (Phelps).
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Florian Holsboer for the American College of Neurophsychopharmacology: Neuroendocrinology of Mood Disorders
National Institute of Mental Health: Bipolar Disorder
James R. Phelps, M.D. Thyroid and Bipolar Disorder
Robertas Bunevičius; Arthur J. Prange Jr. Thyroid Disease and Mental Disorders: Cause and Effect or Only comorbidity?