A thyroid disorder can lead to depression. It is unfortunate that this side effect has been misdiagnosed by so many psychiatrists. How well do psychiatric practitioners know their medical facts? Would they recognize a thyroid disorder when one is presented? Apparently early 20^th century medical texts described depression as a major indicator of hypothyroidism. (A thyroid condition wherein this gland is not producing enough hormone) In other words, this is not a newly observed fact.

Psychiatrists have been quick to diagnose depression as a “mental disease” without a single definitive test. Yet there are medical tests for a thyroid disorder. However, what is thought of as normal thyroid level is somewhat controversial. There are those who fall within the “subclinical” definition of low thyroid and these people sometimes fall in the cracks and are lost, literally and figuratively. They may be the most susceptible to a psychiatric diagnosis of “depression.” 
Experts in Israel discovered that one half of patients with non-psychotic major depression do not respond to antidepressant treatment. When T-3 (a thyroid hormone) was given to the patients, 62% of female patients responded. 
WebMD points out that hormones produced by the thyroid gland can be related to depression. Some symptoms of low thyroid that are may be diagnosed as depressive symptoms are fatigue, weight changes, trouble sleeping and irritability. According to this same article, close to 20 million Americans may have a malfunctioning thyroid, and women are up to eight times more likely to have a thyroid disorder than their male counterparts.
Dr. Todd B. Nippold of the Mayo Clinic’s online website declares “Yes, thyroid disease can affect your mood — primarily causing either anxiety or depression. Generally, the more severe the thyroid disease, the more severe the mood changes. “
What does Dr. Nippold suggest for treating a thyroid disorder? In his words, “Appropriate treatment — such as medication that blocks your body’s ability to produce new thyroid hormone or replaces missing thyroid hormone — usually improves both emotional and physical symptoms caused by thyroid disease.”
Gabe Mirkin, M.D. says this about the use of T3 thyroid hormone in treating depression:
“Exciting research shows that the thyroid hormone called T3 can help treat depression (1,2,3). Psychotherapy often fails to control depression… The dominant theory today is that depression is caused by low brain levels of the neurotransmitters, serotonin and norepinephrine. The drugs such as Paxil, Prozac and Zoloft that treat depression are supposed to raise brain levels of these neurotransmitters. Doctors can also raise brain levels of serotonin by prescribing pills containing T3, a hormone produced by peripheral tissue from T4, which is produced by the thyroid gland.”
Thankfully, there are medical doctors who take the time to properly diagnose the physical cause of a person’s depression. And unlike a psychiatrist who can offer nothing better than dangerous, expensive anti-depressants and anti-psychotics, a good M.D. can help his or her patient properly balance their thyroid hormone levels to handle the uncomfortable side effects, which can include depression. A thyroid disorder, to be properly diagnosed and handled, should be treated by a medical (not psychiatric) doctor.
Quote from Dr. Nippold:http://www.mayoclinic.com/health/thyroid-disease/AN00986
Web MD:http://www.webmd.com/depression/guide/depression-the-thyroid-and-hormones
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Quote from Dr. Mirkin:http://www.drmirkin.com/morehealth/G171.htm