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.
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