At least 13 million Americans suffer from thyroid disorders, and in more than 80% of cases, the problem is an underactive thyroid gland — hypothyroidism. The condition is more common in women, and the rate rises with age, reaching 20% in women over 65. The interest for mental health is that thyroid deficiency may be associated with cognitive and emotional disturbances, and thyroid hormones may be useful in the treatment of depression.
Sitting at the base of the throat, the thyroid gland produces hormones that regulate basal metabolic rate, the speed at which our bodies burn food for energy. The thyroid gets its directions from the hypothalamus, at the base of the brain, by way of the pituitary gland. On a signal from the hypothalamus, the pituitary sends thyroid-stimulating hormone (TSH) into the bloodstream. It travels to the thyroid gland and causes the release of thyroxine (T4), which is partly converted into triiodothyronine (T3). Through a feedback mechanism, the hypothalamus determines when levels of T4 and T3 are low and alerts the pituitary to supply more TSH.
In a person with hypothyroidism, the thyroid gland does not fully respond to TSH, so levels of T3 and T4 remain low while TSH accumulates in the blood. The most common cause is an autoimmune disease, Hashimoto's thyroiditis, but the symptoms can also result from an infection, from cancer, or from treatment of an overactive thyroid (hyperthyroidism) with surgery, radiation, or medications.
Clinical hypothyroidism is identified by an abnormally high level of TSH and abnormally low levels of thyroid hormones. It is treated with a synthetic form of thyroxine, taken in a pill. Subclinical thyroid deficiency, which has few or no symptoms, is defined as abnormally high TSH with normal thyroid hormone levels. Experts disagree on whether and when it requires treatment.
The symptoms of hypothyroidism are variable and sometimes hard to pin down. They may include fatigue, sluggishness, cold intolerance, weight gain, constipation, muscle or joint pain, thin and brittle hair or fingernails, reduced sexual drive, high blood pressure, high cholesterol, and a slow heart rate. Patients may also have problems with concentration and memory.
Some of these symptoms also occur in depression or other psychiatric disorders, and there may be links between hypothyroidism and depression, although the evidence is conflicting and doubtful.
In an Italian study, 36 women with mild hypothyroidism performed poorly on neuropsychological tests and psychological rating scales. After six months of standard treatment with thyroxine, their mood and verbal fluency improved.
But findings have been inconsistent, especially in studies with larger numbers of participants. In one such survey, Canadian researchers found that the only psychiatric disorder associated with thyroid disease was social anxiety disorder (social phobia). In a study of more than 300 people over age 60 who came to internal medicine and psychiatry clinics, some of them for depression and others for symptoms suggesting abnormal thyroid activity, researchers found a high rate of depression among those with subclinical hypothyroidism but not those with clinical hypothyroidism.
So the influence of thyroid deficiency on mental health remains uncertain. Findings may conflict because studies have selected patients and evaluated depressive symptoms and thyroid function by different standards.
There's better evidence that thyroid medication may be helpful for depressed patients, even those with normal thyroid function. Canadian researchers found that added thyroxine helped patients with major depression who did not respond to selective serotonin reuptake inhibitors.
Researchers at Massachusetts General Hospital in Boston administered either thyroxine or the mood stabilizer lithium to 142 patients whose depression had not improved despite earlier treatment. About 25% of those taking thyroxine improved, compared with 16% of those taking lithium.
Examining all the findings so far, an expert panel has concluded that there is not enough evidence to associate TSH levels with psychiatric symptoms or to recommend thyroxine treatment for depressed patients. But there may be just enough evidence to explore these possibilities further — and to recommend tests of thyroid function in seriously depressed patients.
August 2007 update