There has been a great deal of misunderstanding about the connection between thyroid hormone and weight loss. Produced in the thyroid gland, thyroid hormone is the master metabolic control mechanism. A lack of thyroid hormone (a condition called hypothyroidism) is connected to weight gain, as well as dry hair and skin, fatigue, and sluggishness. Overweight people may want to check their thyroid levels to make sure they aren't lacking thyroid hormone. If they are, a physician may prescribe thyroid hormones to correct the condition.

In the 1960s and 1970s, the connection between hypothyroidism and weight gain caused some people to assume they could speed up their metabolism and lose weight by using supplemental thyroid hormones. This led to an abuse of thyroid hormone as people created an artificial state of excess thyroid hormone (a condition medically known as hyperthyroidism). Hyperthyroidism can cause weight loss as well as irregular heartbeats, sweating, and tremors. Although people taking supplemental thyroid hormones may have lost weight, they were losing lean muscle mass in addition to undesirable body fat (Braunwald et al 2001).

Today our understanding of the relationship between thyroid hormone and weight loss is more complete. It works like this: when calorie intake is drastically reduced, the activity of an enzyme called 5'-monodeiodinase is reduced; 5'-monodeiodinase is necessary to convert the thyroid hormone T4 into T3. As a result, the levels of T3 drop (Merimee et al 1976; Carlson et al 1977; Beer et al 1989; Wadden et al 1990). T3 is the stronger form of thyroid hormone. The connection is especially valid when it comes to a reduction in carbohydrate calories:

As little as 50 g of glucose reverses the change in T3 (Burman et al 1979)

Replacement of carbohydrate with fat results in thyroid hormone changes typically observed during times of starvation (Danforth et al 1975; Azizi 1978)

Protein consumption improves the rate of T3 generation more than carbohydrate consumption (Harris et al 1978)

Therefore, consuming more carbohydrate calories during dieting can counteract the drop in T3 associated with dieting.

Alternatively, decreased T3 levels can be directly replaced. Some older clinical studies testing this theory were promising.

However, later studies showed that direct T3 supplementation by dieters was connected with muscle wasting (Gardner et al 1979; Vignati et al 1978). During fasting, administration of large doses of T3 caused even more severe muscle wasting (Carter et al 1975).

More recent studies suggest that using very low doses of replacement thyroid hormone during dieting, once the body has switched over from carbohydrate burning to fat burning, may not be associated with muscle breakdown (Nair et al 1989; Byerley et al 1996, Pasquali et al 1984).