Serum myoglobin in primary hypothyroidism and effect of L-thyroxine therapy.

Serum myoglobin (Mb) concentrations have been measured by radioimmunoassay in normal subjects, in hypothyroid patients examined 20 days after withdrawal of thyroid hormone therapy ("short-term" hypothyroidism), and in untreated hypothyroids ("long-term" hypothyroidism). In "short-term" hypothyroids, serum Mb levels were significantly higher (p less than 0.01) than those observed in normal controls, although only a minority or patients showed elevated Mb levels. In "long-term" hypothyroids, serum Mb concentrations were significantly higher than that found in normal controls (p less than 0.0001) and in "short-term" hypothyroids (p less than 0.001). In "long-term" hypothyroidism a significant inverse correlation was found between serum thyroid hormones and Mb, whereas no similar correlation was observed in "short-term" hypothyroidism. The administration of progressively increasing doses of L-T4 untreated hypothyroids was followed by normalization of serum Mb, but serum TSH levels still remained elevated. These data indicate that the duration and the severity of hypothyroidism are important factors in the rise of serum Mb, and that the normalization of serum Mb is faster than that of serum TSH and requires less L-T4 than that needed for normal TSH secretion.