Objectives: To assess the pattern of change in serum myoglobin concentration in subjects with thyroid dysfunction. Methods: Serum samples were selected from 150 subjects with suspected thyroid disorder who were referred to the Royal Hospital, Muscat, Oman. The subjects were 35 males and 115 females, aged 14-56 years with mean ± SD of 34.3 ± 12.7 years. They were classified on the basis of thyroid stimulating hormone (TSH) and free thyroxine (FT4) into 3 groups, each consisting of 50 subjects: hypothyroid, hyperthyroid, and euthyroid subjects. Results: The mean serum myoglobin concentration was higher in hypothyroid patients compared to hyperthyroid and euthyroid subjects (mean ± SD was 38.5 ± 23.1 µg/L in hypothyroid; 18.1 ± 7.0µg/L in hyperthyroid; 17.4 ± 5.7µg/L in euthyroid). There was a significant difference in myoglobin concentration between hypothyroid and euthyroid groups (F = 36.1, p< 0.001), however, there was no significant difference between the hyperthyroid and euthyroid groups. When the mean ± 2SD for myoglobin in euthyroid subjects was calculated, the reference range was 6-29 µg/L. Of the hypothyroid subjects, 29 (58%) had high myoglobin and 21 (42%) had normal myoglobin level. No significant correlation was noticed between TSH or FT4 and myoglobin in all studied subjects. Conclusion:Raised serum myoglobin may be observed in patients with hypothyroidism. Hence hypothyroidism should be considered in the differential diagnosis of patients with raised serum myoglobin concentration
