Published Nov 11 7:52am in connect.mayoclinic.org
Tara Schmidt, Mayo Clinic registered dietitian nutritionist

The thyroid is a butterfly-shaped gland located in the front of the lower neck. The gland produces thyroid hormones which travel in our blood to every tissue in our body. Thyroid hormone helps control our metabolism. This means that is can influence how fast we burn calories.  If your thyroid does not produce enough thyroid hormone, this condition is called hypothyroidism.

Hypothyroidism is associated with many symptoms including constipation, frequently feeling cold, fatigue, and muscle aches. The metabolic slowing associated with hypothyroidism can also cause weight gain – mostly due to excess accumulation of salt and water. The weight gain associated with hypothyroidism is generally quite modest and in the region of 5-10lbs. We typically do not see a very large weight gain due to hypothyroidism alone. This is because there are many other hormonal, genetic, dietary, lifestyle and environmental factors that also influence our body weight.

Importantly, the extent of weight gain is related to the severity of the hypothyroidism. Therefore, if your blood tests show that your thyroid hormone levels are severely low, one can expect weight gain at the upper end of the given range, and vice versa. When weight gain is due to hypothyroidism, there are generally other symptoms of hypothyroidism present also.

When hypothyroidism is diagnosed and treated appropriately with thyroid hormone, metabolism returns to normal.  Once treated, the weight gain that is due to hypothyroidism is typically lost and the ability to gain and lose weight is the same as in people who do not have thyroid problems. It is important to note that thyroid hormone should not be used as a weight loss treatment in individuals with normal thyroid function.