The thyroid gland produces both thyroxine (T4) and triodothyronine (T3) which regulate metabolic rate. The levels of these hormones must be tightly controlled in order to prevent an excess or a deficiency in the body and this is usually achieved through feedback loops, which adjust the level of thyroid hormone synthesis depending on its concentration in the blood.
Of course, these regulatory mechanisms can sometimes break down, either due to a problem with the thyroid gland itself, an autoimmune disease which attacks the thyroid gland, or problems with the hypothalamus or pituitary gland which indirectly affect thyroid hormone synthesis through the release of hormones, amongst other reasons. When this occurs, levels of thyroid hormone either increase or decrease and this causes a wide array of symptoms.
When considering symptoms of thyroid gland disease it is useful to appreciate their function within the body. As thyroid hormones increase metabolic rate, an excess in the circulation will cause symptoms that reflect this, such as tachycardia, irritability and restlessness, anxiety, weight loss and muscle weakness. Likewise, a deficiency of thyroid hormone will do the opposite, causing bradycardia, lethargy and depression and weight gain.
For a concise summary of thyroid disease, please see the following disease summary. As always, this is not exhaustive, but gives a picture of how hyper- and hypothyroidism can present and what types of treatment are available.