Impact of thyroxine-binding globulin on thyroid hormone economy during pregnancy

Pregnancy-associated changes in thyroid hormone economy are well-established and are of significant clinical relevance to women with established hypothyroidism because they usually result in increased thyroxine dose requirements by these women. Studies suggest that elevations in serum thyroxine-binding globulin (TBG) have the most influence on this increased need for thyroxine, although the exact contributions by TBG rises and by other mechanisms is as yet unclear. We report the case of a 42-year-old woman, with both established primary hypothyroidism and TBG deficiency, who we have now managed through two full-term pregnancies. The patient was noted to have a baseline TBG that was approximately 30% of the average baseline level reported for non-TBG-deficient individuals. Her TBG levels were induced by pregnancy, although the absolute increase of 1.0 mg/dL was only half the increase usually associated with pregnancy. Despite the patient's low baseline TBG level and her blunted pregnancy-associated TBG induction, her absolute and relative pregnancy-associated increases in thyroxine replacement dosage mirrored those found in non-TBG-deficient, hypothyroid women. Thus, our limited study suggests that an increase in TBG concentration is not the key determinant for the increase in thyroxine requirement in pregnancy.