Thyroid Dysfunction and Mental Health
For many years, doctors have found that patients with depression, anxiety and other psychiatric disorders frequently had abnormal blood levels of thyroid hormone. Depression has been so strongly associated with hypothyroidism that was commonly screened for as a first course of action. Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone. Symptoms include fatigue, constipation, hair loss, dry skin, increased sensitivity to cold, difficulty losing weight, and depression.
Beginning in the 1950s, a large body of research conducted at psychiatric hospitals and nursing homes revealed the association between hypothyroidism and depression. With the advent of antidepressants, doctors found that adding pharmaceutical or glandular thyroid hormones typically hastened a depressed patient's recovery, even in cases when the person had not been diagnosed with hypothyroidism.
Studies have found that people with bipolar disorder frequently have abnormal thyroid function. Bipolar patients with rapid cycling who have co-occurring thyroid problems often require thyroid medication. This is particularly true for people who take lithium, because it can cause low thyroid levels.
Postpartum depression, also known as PPD, is often found in conjunction with postpartum thyroid disease. Studies have found that women who test positive for thyroid antibodies can be at increased risk of developing PDD.
In an effort to understand why thyroid function is so strongly linked to mood disorders, scientists began to believe that there was a neurological component. After research indicated that drug treatment of thyroid dysfunction caused improvements in mood, memory and cognition, investigators thought that hypothyroidism might be associated with an increased risk of cognitive impairment, Alzheimer's, and other forms of dementia. Based on this hypothesis, researchers studied the memory and cognitive skills of patients with hypothyroidism both before and after treatment.
In a 2006 study, researchers in China used brain imaging with patients with hypothyroidism and claimed that they saw improvements in memory and executive function after six months of drug therapy. However, other research indicated that higher thyroid levels were associated with a decrease in mental acuity. It appeared that in older individuals, higher levels of thyroid hormones were linked to poor mental performance and an increased likelihood of cognitive decline.
A 2012 review of several studies had more mixed results. The review concluded that further research was needed in order to determine if hypothyroidism was associated with significant mood and cognitive impairments. In 2013, researchers from Houston's University of Texas Medical School examined 1,904 people with hypothyroidism between 70 and 89 years of age in an effort to determine the neurological effects of hypothyroidism. The researchers found that memory and thinking problems happened with a similar rate of occurrence whether or not the patient had hypothyroidism. They concluded that there was no association between hypothyroidism and cognitive impairment or mild dementia.
Based on this new study, the way in which hypothyroidism affects mental health becomes more intriguing. If neurological impairment is taken off the table, it appears that the depression that accompanies hypothyroidism may be due largely to fatigue, one of the most common reported symptoms.
Parsaik AK, Singh B, Roberts RO, et al. Hypothyroidism and risk of mild cognitive impairment in elderly persons: a population-based study. JAMA Neurology. 2013 Dec 30. doi: 10.1001/jamaneurol.2013.5402. [Epub ahead of print]