Impact of Variations in Thyroid Function on Metabolic Function and Body Composition

blue glove holding a "thyroid test" labeled vialResearch has shown that people who have thyroid disease have changes to their metabolic function. However, it is not exactly clear whether variations in thyroid function within the normal range directly impact patients’ metabolism.

To investigate this more deeply, researchers from Oregon Health & Science University in Portland, OR, recruited 67 patients for their study. The participants were healthy age- and body mass index (BMI)-matched women who had no known thyroid disease (n=14) or who were currently taking levothyroxine medication (L-T4) for hypothyroidism (n=53).

Results of their study were published in the paper “Effects of variations in thyroid function within the normal range on metabolic function and body composition.” The paper was presented at the 81st Annual Meeting of the American Thyroid Association.

All study participants had thyroid stimulating hormone (TSH) levels in the normal range. Various measurements for each participant were taken, including resting energy expenditure (REE), respiratory quotient (RQ), and thermic effect of food (TEF) by indirect calorimetry. Body composition was also measured using dual-energy x-ray absorptiometry (DXA).

The participants were divided into 2 groups: low normal TSH (0.42 to 2.49 mU/L; n=45) and high-normal TSH (2.50 to 4.87 mU/L, n= 22).

The research team used a regression model, adjusting for age and BMI where appropriate. They found that there were no differences in any metabolic measures between the 2 groups.

Researchers noted that by correlation analysis, log free T3 was directly related to REE (Pcorr=0.36; p<0.005) as well as percentage of body fat (Pcorr=0.40; p<0.001), but log free T4 was inversely related to TEF (Pcorr=-0.29; p=0.02), as well as lean body mass (Pcorr=-0.22; p=0.08).

It was also noted that inclusion of participants who received only L-T4 did not substantially alter the study’s results.

In addition, using sensitive techniques, there were no differences in measures of energy metabolism, nutrient oxidation, or body composition between the women with high-normal and low-normal TSH levels. However, the link between free T4/T3 as well as some of these measures implies that it is possible that minor differences in thyroid function in the normal range may have a small effect on metabolic function. Researchers concluded that further investigation is needed to accurately determine if these effects are of clinical significance.

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