May 15, 2012 (Florence, Italy) — Weight changes might be linked to changes in thyroid hormone levels, or vice versa, according to a study of metabolic parameters and body composition in men 25 to 45 years of age.

Greet Roef, MD, from the University Hospital in Ghent, Belgium, and colleagues presented the findings here at Joint 15th International Congress of Endocrinology and 14th European Congress of Endocrinology.

"This study suggests that body composition (i.e., fat mass) might influence circulating thyroid hormone levels, even in subjects who are not [extremely] obese," Dr. Roef told Medscape Medical News. However, she pointed out that because of the study design, it was not possible to establish the direction of the observed associations (i.e., whether variation in thyroid hormone levels leads to an altered body composition or whether a less favorable body composition leads to changes in circulating thyroid hormone levels).

According to the researchers, "thyroid disorders affect metabolism and body composition; however, data have been conflicting on whether this is also the case for thyroid hormone levels in the euthyroid range."

They sought to investigate the relation between indices of thyroid status and both body composition and metabolic parameters in 941 healthy euthyroid men, 25 to 45 years of age, with a median body mass index (BMI) of 24.6 kg/m².

Patients were excluded from the study if they had a history of thyroid disease or the presence of antithyroid antibodies.

The researchers measured parameters of body composition and metabolism, as well as total (TT3, TT4) and free thyroid hormones (FT3, FT4), thyroid-stimulating hormone (TSH), thyroxin-binding globulin (TBG), and reverse T3 (rT3).

Higher BMI, fat mass, and serum leptin were associated with higher circulating levels of FT3, TT3, and TBG (P < .02 for all), whereas FT4 and TT4 were significantly associated with fat mass and serum leptin (< .005), but not BMI.

In contrast, higher levels of T3 and TBG were associated with lower insulin sensitivity, assessed by HOMA-IR (= .0001); there appeared to be no link between TSH levels and body composition or metabolic parameters.

"Thyroid hormone levels were positively associated with fat mass and leptin, and inversely associated with indices of muscle mass and insulin sensitivity in this population of healthy young men with well-characterized euthyroidism," Dr. Roef and colleagues conclude. "The direction and underlying mechanisms of these robust and coherent associations are not known."

"Our results point to changes in thyroid hormone levels being secondary to alterations in body composition, but we cannot prove this claim in this cross-sectional study," Dr. Roef reported.

"Furthermore, the absence of an association with TSH is remarkable, especially since our associations with free and total thyroid hormones are very significant," she added.

According to Dr. Roef, clinicians should keep in mind that weight changes can influence circulating thyroid hormones.

Independent commentator Robin Peeters, MD, PhD, from the Erasmus Medical Center, in Rotterdam, the Netherlands, noted that these findings are "surprising. I would have expected an opposite relationship."

"Nevertheless, the data are convincing, and future studies should confirm this in independent cohorts," Dr. Peeters told Medscape Medical News. "We should elucidate the mechanisms underlying these effects before we can make conclusions about this," he added.

The study was not commercially funded. Dr. Roef and Dr. Peeters have disclosed no relevant financial relationships.

Joint 15th International Congress of Endocrinology (ICE) and 14th European Congress of Endocrinology (ECE): Abstract OC2.3. Presented May 7, 2012.

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