Muscle Dysfunction in Subclinical Hypothyroidismd

In a recent paper, Monzani et al. (1) showed that blood lactate levels increase more than normally during exercise in patients with subclinical hypothyroidism (sHT). They concluded that muscle energy metabolism is impaired in this condition. We have personal data supporting this hypothesis. Thirteen patients (5 men, 8 women, ages 41–88 yr, mean 72.5 yr ± 14.6 sd), not admitted for thyroid disease and without any history of thyroid pathology, were recruted at admission in a general referal hospital on the basis of high blood levels of thyrotropin (TSH, ultrasensitive ICMA assay, Ciba Corning) and normal levels of free thyroid hormones (free thyroxine, FT4, and free triiodothyronine, FT3, RIA, Amerlex-Mab kit, Orange Medical). Serum creatine kinase (CK) activity (CK kit, Behring) was measured in these patients and history, physical examination, electrocardiogram, and CK isoenzyme analysis allowed us to exclude CK elevation due to heart or brain damage. As shown in Fig. 1, CK levels were at the upper normal value or above in 6 out of the 13 patients. We observed a positive correlation between CK and TSH (r= 0.77, P < 0.01) and an inverse correlation between CK and FT4 (r = 0.55, P = 0.05), whereas the correlation between CK and FT3 (not shown) did not reach statistical significance (r = 0.28). It is admitted that serum CK of skeletal muscle origin is increased in patients with overt hypothyroidism (2–4), a fact we have also observed in such patients (data not shown). Our results suggest that this may also be the case to some extent in subclinical hypothyroidism. Monzani et al. thus demonstrated muscular abnormalities during exercise in sHT apparently related to the duration of this condition. In our cross-sectional analysis in patients at rest the statistical significance of the relationship between a biological parameter easy to measure, serum CK activity, and the degree of sHT sustains the hypothesis of muscular impairment. We agree with these authors in suggesting that sHT may require earlier treatment than previously thought. Correlation between serum creatine kinase (CK) concentrations and the circulating levels of thyrotropin (TSH) and free thyroxine (FT4). CK levels are represented on a logarithmic scale. The dotted lines indicate the range of normal values. Correlation between serum creatine kinase (CK) concentrations and the circulating levels of thyrotropin (TSH) and free thyroxine (FT4). CK levels are represented on a logarithmic scale. The dotted lines indicate the range of normal values.