Changes in Thyroid Function Tests During Phenobarbital Treatment in Late Pregnancy

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Abstract
The effect of phenobarbital treatment in late pregnancy on thyroid function tests was investigated in 16 women. Ten women in late pregnancy served as controls. Phenobarbital treatment for two weeks was associated with a significant decrease in serum thyroxine (T4) level and in the free thyroxine index (T3U × T4) of 18% and 16%, respectively. These changes were associated with a significant increase in serum thyrotropin concentration of 15%. γ‐Glutamyl transpeptidase activity increased, and serum bilirubin concentration decreased during phenobarbital treatment. The changes in the thyroid function test might reflect the effect of phenobarbital on thyroid hormone metabolism. The changes may have diagnostic importance in borderline cases of thyroid disease.