The prospective relationship between low muscle mass and thyroid
hormones among 198,069 euthyroid men and women; comparing different
definitions of low muscle mass
Objective: The impact of thyroid hormones within normal range on muscle
mass remains unknown. We examined the association between new onset of
low muscle mass (LMM) and thyroid hormones among euthyroid men and women
with three different definitions of LMM in prospective cohort study.
Methods: We performed a cohort study of 198,069 Korean adults (mean age
of 39.2 years), free of LMM at baseline, who participated in a repeated
screening examination and were followed-up annually or biennially for up
to 6.3 years. Thyroid-stimulating hormone (TSH), free triiodothyronine
(FT3) and free thyroxin (FT4) levels were measured by an
electrochemiluminescent immunoassay. Muscle mass was assessed using a
bioelectrical impedance analyzer. LMM was defined as the appendicular
skeletal muscle mass (ASM) by body weight (ASM/weight, LMM-W), height
squared (ASM/height2, LMM-H) and BMI (ASM/BMI, LMM-B) of one standard
deviation below the sex-specific mean for young reference group.
Results: During a median follow-up of 3.1 years (interquartile range,
2.0-4.1 years), new-onset LMM-W, LMM-H, and LMM-B occurred in 17,856
(incident rate, 27.8 per 1,000 person-years), 8,307 (incident rate, 13.4
per 1,000 person-years), and 13,990 participants (incident rate, 24.5
per 1,000 person-years) in each. In euthyroid men, FT4 was inversely and
FT3 positively associated with incident LMM-W in a dose-response manner.
TSH and FT4 had inverse dose-response relationship with incident LMM-B.
Incident LMM-H of euthyroid men has no apparent associations with any
thyroid hormones. Euthyroid women had no dose-response relationship
between thyroid hormones and any definition of LMM. Conclusions: Among
euthyroid men, FT4 had inverse dose-response association with new onset
of LMM defined with weight (LMM-W) and BMI (LMM-B). Height squared LMM
(LMM-H) had no apparent relationship with any thyroid hormones.
Euthyroid women had no dose-responsive association between thyroid
hormones and incident LMM. Key words: appendicular skeletal muscle mass,
low muscle mass, thyroid hormones, Cohort study