Of the estimated 20 million people in the United States who have a problem with their thyroid gland, up to 60 percent don’t know it.
In many people, symptoms of depression may offer a clue in detecting an underactive or overactive thyroid. Even minor fluctuations in thyroid activity can increase the risk of developing depressive symptoms, recent research has found, especially in older adults.
The link to mood
Precisely how the thyroid can affect mood is not fully understood. Thyroid hormone receptors are found throughout the brain, including areas linked to mood disorders, so it’s possible that changes in thyroid hormone levels could affect mood.
It is also believed that thyroid hormones interact with mood-regulating chemicals in the brain called neurotransmitters, particularly serotonin, norepinephrine, and dopamine.
People with an underactive thyroid (hypothyroidism) often experience sad mood, cognitive problems, apathy, and fatigue. Some studies that examined the link between high thyroid-stimulating hormone (TSH) levels—which suggest hypothyroidism—and depressive symptoms have had mixed results.
However, a study of approximately 1,300 middle-aged women, published in the Journal of Affective Disorders in 2009, found that those with clinically diagnosed hypothyroidism (a high TSH level plus a low level of the hormone thyroxine, or T4) were nearly nine times more likely to develop symptoms of depression than women who did not have hypothyroidism.
Hyperthyroidism (overactive thyroid) usually causes symptoms such as nervousness, irritability, or anxiety. However, in people older than 65, it can cause symptoms of depression, including depressed mood, apathy, or lethargy.
There’s some evidence that having a thyroid gland that is even slightly less active can cause mood problems. “Subclinical” hypothyroidism—defined as having normal thyroid hormone levels but a slightly high TSH level—has been linked to depression. Subclinical hypothyroidism is being increasingly diagnosed in people older than age 55, as they are most likely to develop mood problems.
Research also suggests that even a slight increase in thyroid activity may increase the risk of developing depressive symptoms.
A study published in 2014 in the Journal of Clinical Endocrinology & Metabolism examined how often depressive symptoms occurred in people whose TSH levels were at high and low ends of a normal range, measured in milliunits per liter (mU/L). (In this study, a normal TSH range was defined as 0.3 mU/L to 4.0 mU/L).
Compared with people who had TSH levels in the highest normal range (1.6 mU/L to 4.0 mU/L), those in the lowest normal range (0.3 mU/L to 1.0 mU/L, indicating a slightly overactive thyroid) had more symptoms of depression and were more likely to develop depressive symptoms if they didn’t have them when the study began.
Correcting the thyroid
The good news is that for people for whom a thyroid disorder is to blame for symptoms of depression, successfully treating the thyroid problem can eliminate those symptoms.
For people with hypothyroidism, the usual treatment is to take a synthetic form of T4 (called levothyroxine or L-thyroxine) to replace the T4 the thyroid isn’t producing on its own.
Hyperthyroidism is treated with medication, radioactive iodine, or, in rare cases, surgery, depending on the underlying cause and the person’s age. In most people, once the thyroid imbalance is corrected, the mood symptoms are reversed.
If you develop symptoms of depression—or any other symptoms that might be the result of hypothyroidism or hyperthyroidism—ask your doctor about checking your thyroid.
Be sure to tell him or her about any medications you are currently taking, whether prescribed or over-the-counter, and any other symptoms you have recently experienced.
Is it depression or grief? Learn how to tell the difference.
Jeff Bauer is a healthcare journalist with expertise in psychiatry. He has served as editor of Current Psychiatry, a leading peer-reviewed clinical journal for psychiatrists and other mental health practitioners, and as educational content director for the U.S. Psychiatric and Mental Health Congress, the nation’s leading independent mental health continuing education conference.