Although hypothyroidism is associated with some weight gain, surveys of obese individuals show that less than 10% are hypothyroid. Discrepancies have been reported in some studies, but in untreated obesity, total and free T4, total and free T3, TSH levels, and the TSH response to TRH are normal. Some reports suggest an increase in total T3 and decrease in rT3 induced by overfeeding. Treatment of obesity with hypocaloric diets causes changes in thyroid function that resemble sick euthyroid syndrome. Changes consist of a decrease in total T4 and total and free T3 with a corresponding increase in rT3.
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