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No negative effect of levothyroxine therapy on energy expenditure, body composition
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Energy expenditure and body composition in women was not adversely affected by levothyroxine suppression therapy.
However, despite thyroid-stimulating hormone levels within the reference range, levothyroxine replacement therapy was linked with lower resting energy expenditure, according to the researchers.
Mary H. Samuels, MD, of the division of endocrinology, diabetes and clinical nutrition at Oregon Health & Science University in Portland, Oregon, and colleagues evaluated three groups of women (women receiving chronic TSH-suppressive levothyroxine doses, n = 26; women with euthyroidism receiving chronic levothyroxine replacement doses, n = 80; and untreated euthyroid controls, n = 16) to determine if TSH-suppressive levothyroxine doses alter energy expenditure or body composition.
Resting energy expenditure, substrate oxidation, thermic effect of food, physical activity energy expenditure, caloric intake and body composition were measured for all participants.
Participants with euthyroidism treated with levothyroxine replacement had 6% lower resting energy expenditure per kilogram of lean body mass compared with the levothyroxine suppressed group and 4% lower than controls. Similar resting energy expenditure levels were found among the levothyroxine suppressed group and controls (P = .68).
A positive relationship was found between serum free triiodothyronine levels and resting energy expenditure (P = .03) but not free thyroxine or TSH levels. No differences were found between the three groups for substrate oxidation rates, thermic effect of food parameters, body composition, dietary intake measures or physical activity.
“We found that [levothyroxine] suppressed subjects had similar measures of energy expenditure and body composition as healthy control subjects, indicating that minimal [levothyroxine] suppression therapy does not produce adverse effects on metabolic function,” the researchers wrote. “On the other hand, [levothyroxine] euthyroid women had lower [resting energy expenditure] than [levothyroxine] suppressed women, and a trend toward lower [resting energy expenditure] than healthy controls. This group also had lower mean [free T3] levels, raising the intriguing possibility that low-dose [levotriiodothyronine] therapy might benefit the metabolic profile of these women.” – by Amber Cox
Disclosure: The researchers report no relevant financial disclosures.
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