Weight gain in hypothyroidism

The relationship between the thyroid and weight

The thyroid hormones regulate a portion of metabolism. Metabolism is determined by measuring the amount of oxygen used by the body over a period of time. If the measurement is made when person is resting, this is called the basal metabolic rate (BMR). Patients who have hypothyroidism can have low BMR rates. There are many basal metabolic rate calculators on the internet.  

BMR and hypothyroidism

Patients with hypothyroidism generally experience alterations in body weight that are due to energy intake or energy use, leading to a loss or accumulation of body fat or a change in body water content. The basal metabolic rate is the sum of the total daily energy expenditure that is generally in the range of 60%, which includes the energy required for physical activity (approximately 10 to 30% or more), or from the thermogenic effect of food that includes another 10% or more.  Since thyroid hormones are a major determinants of the basal metabolic rate, it likely causes a correlation between biochemical thyroid function and the body mass index (BMI). Still, there is another possibility for the weight increase with hypothyroidism, which is a decreasing level of daily spontaneous activity.

Since the BMR is decreased in patients with hypothyroidism there can be associated weight gain. It is usually more severe weight gain with more severe hypothyroidism. Since the causes of weight gain in the setting of hypothyroidism is so complex, generalizing that weight gain can be reversed with the use of thyroid medications cannot be made. Massive weight gain associated with hypothyroidism is very uncommon.  If weight gain is the predominant symptom a person has, then it is much less likely that the weight gain is solely due to the thyroid dysfunction, if present. 

How much weight can be lost

Since most of the weight gain in hypothyroidism is due to accumulation of salt and water, once the thyroid is treated, then there can be about a 10% loss in body weight. This is a very small amount, and is usually approximately 5-10 pounds in the average person. Since hypothyroidism usually occurs over a long period of time, is it common to find that there is no significant weight after successful treatment of hypothyroidism. Once the hypothyroidism has been treated to proper ranges by your endocrinologist, then the ability to lose or gain weight is the same as in people that do not have thyroid problems. 

Use of thyroid hormones for weight loss

Thyroid hormones have been used to assist in weight loss in the past. At Houston Thyroid and Endocrine we do not encourage this approach since it is dangerous to take excess thyroid hormone.  A patient can take too much thyroid hormone which may cause weight loss, but also cause negative side effects: loss of muscle protein, cardiac disease, bone disease, and other metabolic problems. 

Karmisholt et al. Weight loss that occurs during levothyroxine treatment of hypothyroidism is predominantly due to loss of excess body water accumulated during the development of myxedema  J Clin Endocrinol Metab 2010.jc.2010-1521
Parving HH, Hansen JM, Nielsen SL et al. Mechanisms of edema formation in myxedema--increased protein extravasation and relatively slow lymphatic drainage. N Engl J Med 1979;301:460-5.