Background
Thyroid hormones are important metabolic regulators during exercise as well as at rest, and they affect muscular and cardiorespiratory function. Thus, both patients with hypothyroidism and those with hyperthyroidism can present with profound musculoskeletal weakness. Clinical and subclinical hypothyroidism are characterized by chronotropic incompetence, while treatment with levothyroxine improves cardiovascular variables such as heart rate and cardiac output. The muscles respond vigorously on increases in the thyroid hormone level, which has led to abuse of thyroid hormones as physical performance enhancers by people performing strength-training exercises, despite the well-known catabolic action of excess thyroid hormone (1). Thus, possible benefits are unclear, but they may be related to cardiac stimulation, muscle performance, or both. This study (2) aimed to investigate the association of thyroid function, defined by serum thyrotropin (TSH) and free thyroxine (FT4) concentrations, and physical activity in a large population-based cohort.

Methods
This study (2) used the large prospective population-based Rotterdam cohort study initiated in 1989, which includes 7983 participants ≥55 years and expanded twice up to 2006. It included participants with information on thyroid function variables and physical activity for cross-sectional analyses and repeated measurements of physical activity for longitudinal analyses (the Longitudinal Aging Study Amsterdam [LASA] questionnaire). Baseline data were from 2006 to 2008 and repeated measures from 2008 to 2013.

TSH, FT4, and thyroid peroxidase antibodies were measured from stored serum samples (electrochemiluminescence immunoassay; Roche). Physical activity was evaluated by an adapted validated version of the LASA Physical Activity Questionnaire and assessed by the metabolic equivalent task (MET, defined as 1 kcal/kg/hr). MET-hours were calculated by multiplying the MET value by time (in hours) spent on each specific activity and summing all activities from the 2011 Compendium of Physical Activity per week.

Results
Among the 2470 subjects with baseline characteristics, 1907 also had repeated measurements of physical activity. The mean age was 57.3 years, and 58% were women. The median TSH was 2.04 mIU/L (interquartile range, 1.4–2.79). The mean (±SD) FT4 was 1.21±0.17 ng/dL. The median physical activity was 22.50 MET (interquartile range, 11.56–40.67). Baseline TSH and FT4 concentrations were similar between participants in the cross-sectional and longitudinal analyses, with an average follow-up time of 5 years (range, 2–8).

There was no association of TSH (adjusted mean difference [b], 0.65; 95% confidence interval [CI], –1.67 to 2.98) and FT4 (b, 2.76; 95% CI, –7.15 to 12.66) with the baseline physical activity score. The longitudinal analyses also showed no association of thyroid function with physical activity (TSH b, 1.16; 95% CI, –1.31 to 3.63; FT4 b, –6.63; 95% CI, –17.06 to 3.80) or between TPOAb and physical activity (b, 2.63; 95% CI, –2.58 to 7.84). Also, there was no association of TSH, FT4, or TPOAb with moderate or vigorous physical activity on cross-sectional or longitudinal analysis after adjusting for multiple testing.

Conclusions
This population-based study found no impact from thyroid function on physical activity in middle-aged to old individuals from the general population. Similar results may pertain to patients receiving levothyroxine therapy, which is relevant if the goal of prescribing thyroid hormone therapy is to improve daily physical activity levels in adults with mild or subclinical hypothyroidism.