The relationship between obesity and thyroid hormone is important. It is well known that low thyroid hormone levels can be the cause of obesity (however, not the only cause) and it is one of the most commonly checked laboratory studies in obese patients.


What happens to the thyroid hormone profile after weight loss has begun? This is an important topic as many of our patients are on thyroid replacement therapy and patients need to understand the importance of specialization in correcting the metabolism.


There are mainly two important thyroid hormones produced by the thyroid gland; thyroxine (T4) and 3,5,3’-triidodothyronine (T3). T4 is solely produced by the thyroid gland while T3 comes from two sources; the thyroid gland and from conversion of T4 to T3 in many other tissues.


Every day the thyroid gland produces around 100 mcg of T4. Approximately 10% of the T4 is metabolized per day. 40% of it is converted to T3 which is 3 times more potent than T4.1 Another 40% is converted to reverse T3 which is metabolically inactive. The remaining 20 percent is conjugated with glucuronide and sulfate.


T3 is produced by two ways. 80% of it is produced by peripheral conversion of T4 and the remaining 20 percent is produced by the thyroid gland. The total production rate is 30-40 mcg per day.


Thyroid hormones levels need to be monitored very closely during weight loss, especially when the patient is on thyroid replacement therapy.


As we have seen earlier, the most portent hormone is T3. Around 80% of T3 is produced by peripheral conversion of T4.


A modest weight loss of even 5-10% of original weight causes a decrease in T3. This happens because weight loss has been found to decrease the peripheral conversion of T4 into the more potent T3.2  Patients who receive thyroid replacement therapy should be following the thyroid hormone level need closely during active weight loss and the dose of levothyroxine/Synthroid may need to active adjustment to achieve a normal level.


The Metabolism Clinic is established as the destination for weight loss and reversal of diabetes. Based in Charlotte, North Carolina.