Lower energy expenditure and T3 in hypothyroid women on thyroid hormone replacement

Lower Energy expenditure in women receiving thyroid hormone replacement for hypothyroidism.
In this March 2016 issue on Thyroid an article explores the effects of thyroid hormone replacement therapy on energy expenditure and body composition. This is a cross sectional study and looks just at a group of people at point in time. However the study is admirable in the multiple earnest efforts used to document, diet, activity and calorimetry. In this study they had three groups: A control group of 16 healthy individuals on no thyroid replacement, a group of 80 women receiving chronic thyroid replacement with the levothyroxine for hypothyroidism, and 26 women who get higher levothyroxine doses or TSH-suppressive levothyroxine therapy for thyroid cancer treatment and prevention of recurrence.
Findings: The researchers found that resting energy expenditure per kilogram was the lowest in women that were receiving levothyroxine therapy in contrast to healthy controls that were not receiving any thyroid hormone and those that were receiving higher doses of thyroid hormone for thyroid cancer treatment. The fascinating finding was that even though all women who were hypothyroid and were receiving levothyroxine therapy had normal and similar TSH levels as the healthy control group, this group had significantly lower free T3 levels compared to the normal control group and group on TSH suppressive therapy. Both levothyroxine-treated groups had significantly higher free T4 than control group. The lower free T3 levels found in the hypothyroid group on replacement correlated with a lower resting energy expenditure. They did not find harm to metabolism for the women who were receiving higher thyroid doses for cancer prevention. No weight or dietary differences were found between these 3 groups. So they cannot say with these data the the lower energy expenditure resulted in differences in body composition or weight.

Discussion and my take on this: These data contribute to a growing body of evidence that eventhough TSH is “normal” some people receiving levothyroxine may experience relative tissue hypothyroidism. The study suggests the possibility that low dose T3 therapy may be of benefit to increase energy expenditure and enhance metabolism. We need better ways to replace T3 in patients, current preparations are too short acting and need to be taken multiple times a day. Investment and development of slow release T3 that is reliable and FDA approved would help a whole lot of hypothyroid patients.

Other personal opinion: I am in the minority of board certified endocrinologists that measures free T3 in the setting of hypothyroidism, particularly if people are not well. Neither The Endocrine Society or American Thyroid Association recommended measuring FreeT3 or total T3 for monitoring hypothyroidism at this point, because the tests to date are not great and more data and test reliability accompanies the TSH test.

I disagree strongly with other integrative practitioners in that I do not measure reverse T3. I do not find it useful unless someone is quite sick or in an ICU. I already know rT3 may be higher in people who are very inflamed, on high doses of steroids or beta-blockers, or after starvation, and/or very old. Further giving T3 does not make reverse T3 in these sick individuals any better. Solving what got them sick in the first place is the therapy.

References:
Samuels MH etal. Thyroid,26(3):347-355. DOI: 10. 1089/thy.2015.0345.
Gardener DF et al. 1979 NEJM 300:579-584

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