The butterfly-shaped thyroid gland, part of the body's endocrine system, helps regulate your metabolism. The hormones produced by your thyroid affect vital body functions like heart and respiratory rate, growth, fertility and digestion. Thyroid levels can also have an effect on a person's weight, with an underactive thyroid contributing to weight gain and an overactive thyroid causing weight loss.
Thyroid and Weight
A person's metabolism is regulated by a number of different bodily hormones, among them the hormone secreted by the thyroid gland. A change in thyroid hormone levels can lead to a change in BMR, or basal metabolic rate, which controls a person's energy balance and weight. A low BMR is associated with low thyroid levels and can lead to weight gain, while a high BMR, associated with high thyroid levels, can lead to weight loss.
Hypothyroidism
Hypothyroidism, which results from an underactive thyroid, can lead to weight gain. The weight gain is not necessarily due entirely to excess fat accumulation, but may result from accumulated salt and water. The amount of weight gain varies depending on the severity of the hypothyroidism, but it is rarely a dramatic amount, according to the American Thyroid Association. Weight gain is less likely to be caused by an underactive thyroid if it is the only symptom present from hypothyroidism.
Hyperthyroidism
The BMR of a person with hyperthyroidism is elevated, thus such a patient often loses weight. The amount of weight lost depends on the severity of the overactive thyroid. Increased BMR leads to an increasingly large number of calories required to maintain weight. Unless a person takes in the amount of calories required by his BMR, a loss in weight will result.
Other Thyroid Disorders
There are other medical conditions besides hyperthyroidism that can result in weight loss, including the toxic phase of thyroiditis, an inflammation of the thyroid gland. An excess amount of synthetic thyroid medication can also produce symptoms of hyperthyroidism, including weight loss.
Thyroid Levels and Weight Loss
Doctors use a thyroid stimulating hormone or TSH blood test to determine a person's thyroid levels. Normal levels of TSH are in the range of 0.4 to 2.5. A level in the range of 0 to 0.4 is considered hyperthyroid. Levels between 2.4 and 4.0 indicate that you are at risk of developing hypothyroidism and should be retested within a year. A score above 4 is considered to be mildly hypothyroid, while a person is not considered clinically hypothyroid unless his TSH level is above 10.
A 2007 British study suggests that treating mildly hypothyroid patients can be useful in controlling cholesterol levels and increasing energy, according to the website Oprah.com. However, care is required in the use of synthetic thyroid hormone treatment for weight loss, since muscle protein can be lost along with body fat.
