Design, patients and measurements: The presence of neuromuscular symptoms was ascertained by questionnaire in 33 consecutive patients with subclinical hypothyroidism (sHT) as compared to 44 age- and sex-matched controls. Blood was sampled for PTH, magnesium, phosphate, and total and ionized calcium determination. Patients reporting three or more symptoms were also studied by surface electromyography (sEMG). The study was repeated following a six-month L-T4 course.
Results: Neuromuscular symptoms were significantly more frequent in patients than in controls (P = 0. 0001), and correlated with TSH values (r = 0.52; P = 0.0001). Among patients showing three or more symptoms (n = 11), sEMG documented the presence of repetitive discharges in 8 patients. L-T4 therapy led to a significant improvement of symptoms (P = 0.0001); persistent repetitive discharges were no longer observed. Total and ionized calcium values, always within the normal limits, were significantly lower in patients than controls (P < 0.0001). An inverse relationship was observed between ionized calcium and: TSH values (r = -0.69, P = 0.0001); the number of neuromuscular symptoms (r = -0.53, P = 0.0001). L-T4 replacement induced a significant increase in both total and ionized calcium levels (P < 0.01 and P < 0.0001, respectively).
Conclusions: Neuromuscular symptoms and dysfunction are rather common in subclinical hypothyroidism, and may be associated with abnormalities in serum calcium balance and surface electromyography. The ability of L-T4 treatment to reverse all these changes suggests that subclinical hypothyroidism patients may require early therapy not only to prevent progression to frank hypothyroidism, but also to improve their neuromuscular dysfunction.