The thyroid gland plays a major role in the human body's everyday functions by secreting hormones that control metabolism, according to Harvard Medical School. Because the thyroid gland produces the substances that influence and regulate the body's metabolism, it functions in a manner similar to the conductor of an orchestra. Metabolism determines the rate at which cells are produced, nutrients are absorbed and energy is used, which affects every organ and system in the body.
The pituitary gland monitors the levels of hormones in the bloodstream and sends an activation signal to the thyroid gland when levels of the hormones called T3 and T4 are low. These are the metabolism-regulating hormones produced by the thyroid gland, and when it receives the production signal from the pituitary gland, T3 and T4 are produced and secreted.
When the thyroid gland is unable to produce the called-for amounts of T3 and T4, which results in a slowing down of the body's metabolism, the condition is referred to as hypothyroidism. Some of the symptoms resulting from hypothyroidism are either similar to those of clinical depression or they are capable of causing depression. There are clinicians who believe that some people who are taking anti-depressant medications should instead be examined and tested for hypothyroidism, as reported in the Harvard Medical School newsletter.