The three groups were similar in age (39-41 years old) and BMI (average 25-27 years old). The TSH was similar in the healthy control group and the hypothyroid group on usual replacement (2.13 vs 2.08). The TSH was lower in the suppressive group (0.14 +/- 0.02). Free T4 was highest in the suppressive TSH group, with the next highest levels in the euthyroid replacement group, then lowest in the normal controls. The free T3 values were similar in the suppressive TSH group and the normal controls, and lower in the euthyroid replacement group. Body composition (including lean body mass, fat mass, % fat mass) was not different in the three groups. Dietary intake and physical activity were also the same in all groups. However, REE was lower by 6% in the levothyroxine group with normal TSH values than the suppressive TSH group and 4% lower than normal controls. The REE correlated with free T3 levels, but not TSH or free T4 values.
WHAT ARE THE IMPLICATIONS OF THIS STUDY?
Women on suppressive doses of levothyroxine have similar metabolic profiles to healthy controls. There was no lean muscle mass loss, fat loss, or higher REE with higher than physiologic doses of thyroid hormone. No positive or negative effects on body composition were noted in women on suppressive doses of levothyroxine alone.
The study raises more questions about REE in patients with normal TSH values on levothyroxine replacement. REE was slightly lower in this group, but this group did not have different levels of BMI or body composition, reassuringly. However, the free T3 levels were lower, which may raise further questions about using T3 in combination with free T4. It should be noted that previous studies of combination therapy (T4 plus T3) have not shown any differences in weight from LT4 monotherapy. More studies will be helpful to determine risks and benefits before combination therapy can be widely accepted.
— Julie Hallanger Johnson, MD
ATA THYROID BROCHURE LINKS
Thyroid and Weight: http://www.thyroid.org/thyroid-and-weight/