Thyroid Research Thyroid Research Archive Pregnancy

Increased serum binding only partly explains the need for higher thyroxine doses in pregnant women with hypothyroidism

(March 2004)

The background of the study. Thyroid secretion increases during pregnancy in normal women, and some women with hypothyroidism need higher doses of thyroxine (T4) when they are pregnant. The increased need for T4 during pregnancy is thought to be due to an increase in serum thyroxine-binding globulin (TBG) concentrations. This case report describes a woman with hypothyroidism and familial TBG deficiency who needed an increase in the dose of T4 during pregnancy.

Case report.The patient was a woman who was found to have hypothyroidism and a low serum TBG concentration; she was treated with T4. In 1999 the patient had a normal term pregnancy. At two months’ gestation, while taking 0.112 mg of T4 daily, her serum thyrotropin (TSH) concentration was high, and the T4 dose was raised. Her serum TBG concentration near term was 1.3 mg/dl (normal, 1.7 to 3.6). At term, she delivered a normal baby girl. In 2002 she had another term pregnancy. Her dose of T4 again had to be raised, although her serum TBG concentration remained low. This child was a boy, who had an undetectable serum TBG concentration and a normal serum TSH concentration, confirming the presence of X-linked TBG deficiency in this family.

This woman’s serum TBG concentrations were very low when she was not pregnant. The concentrations increased slightly (but remained low) during pregnancy, but she needed as much of an increase in dose of T4 as do other women with hypothyroidism.

The conclusions of the study. The need for higher doses of T4 during pregnancy in women with hypothyroidism is not caused only by the pregnancy-induced increase in TBG production.

The original article. Zigman JM, Cohen SE, Garber JR. Impact of thyroxine-binding globulin on thyroid hormone economy during pregnancy. Thyroid 2003;13:1169-75.