The thyroid is a gland in the neck that releases important hormones into the blood stream. The hormones have important effects on many cells in the body, and play a significant role in metabolism. Thyroid supplements typically do not cause side-effects if taken appropriately. However, excessive or inadequate supplementation leads to high or low thyroid levels, respectively. This can cause side effects.
Metabolic Side Effects
According to "Robbins and Cotran Pathologic Basis of Disease" by Vinay Kumar MD. Thyroid supplements effect the metabolism. Excessive thyroid supplementation increases the metabolism, and supplementation that is too low decreases the metabolism. This can lead to weight loss or weight gain, respectively, as well as an increased or decreased appetite and utilization of food by the body.
Temperature Regulation Side Effects
Excessive thyroid supplementation can increase the body temperature and cause heat intolerance. In contrast, low levels of thyroid supplementation decrease the body temperature and causes cold intolerance.
Psychological and Neurological Side-Effects
Too much or too little thyroid supplementation has psychological effects and effects on the nervous system. Too little can cause depression and fatigue. This can also have effects on the nerves, causing secondary problems, such as slowing down intestinal movements and leading to constipation. Too much can cause anxiety, hyperactivity, irritability, tremors and diarrhea, according to "Harrison's Principles of Internal Medicine" by Anthony Fauci MD.
Cardiovascular Side Effects
Thyroid supplements also can affect the heart when not optimally dosed. Too much can cause high heart rates and rhythm problems, such as atrial fibrillation. This is a condition in which the top part of the heart quivers erratically instead of beating normally to deliver blood to the lower chambers of the heart, called ventricles. Too little can cause slow heart rates.