Weight loss with thyroid disease is primarily due to a defect in or caused by the thyroid. One of the largest of the endocrine glands in the body, the thyroid is located on the neck, below the adam's apple. It is responsible for bodily processes such as the rate at which the body burns up energy, making proteins, and the body's sensitivity level to other hormones. To address these responsibilities, it produces primary thyroid hormones known as thyroxine or T4, and triiodothyronine or T3. Apart from these two hormones though, the thyroid also makes calcitonin. Thyroxine and triiodothyronine are tasked with regulating the body's metabolism rate, as well as affecting growth and other functions found in the organ systems of the body while calcitonin is important for calcium homeostasis.
Hyperthyroidism, or an overactive thyroid gland, is a clinical syndrome due to an excess amount of thyroxine or triiodothyronine, or both, in the body. Inversely, hypothyroidism is caused by an underactive thyroid gland that does not produce enough of the thyroid hormones needed to regulate bodily processes.
Since the thyroid is responsible for regulating the body's metabolic rate, causing either weight gain or weight loss with thyroid disease. Weight gain can be attributed to the lack of thyroid hormones that cause metabolic rates to slow down, inducing weight gain. And consequently, weight loss with thyroid disease is possible since the increased amounts of thyroid hormones in the body causes an increase in the body's metabolic rates, aiding weight loss. In either situation, weight gain or weight loss with thyroid disease is not a simple matter and would require proper medical attention at the earliest onset. In some cases however, like in postpartum thyroditis as experienced by women within the year after giving birth, hyperthyroidism can correct itself without treatment in a matter of weeks or months.
Treatments for hyperthyroidism could include thyrostatics (inhibits thyroid hormone production), beta-blockers (for treating symptoms), surgery (removal of whole or part of the thyroid), and radioiodine (radioactive isotope in pill or liquid form that destroys the functions of a hyperactive gland). Hypothyroidism treatment on the other hand, could include the use of either animal-derived or synthetic thyroid medication.