Low thyroid hormone levels, or hypothyroidism, can cause a person's metabolism to slow, according to WebMD. A person's metabolic rate also affects his body temperature, heartbeat and his ability to burn calories.
Hypothyroidism is a disease that may be caused by Hashimoto's disease, radiation therapy, radioactive iodine treatment or from using certain medications such as lithium or interferon alpha, according to WebMD. The disease may also be caused by eating too little iodine. The most common symptoms of hypothyroidism include dry hair, hair loss, dry skin, fatigue, sensitivity to cold, slow heart rate and unexplained weight gain. These symptoms all occur due to a low metabolic rate caused by low levels of thyroid hormones.