High serum thyroid hormone concentrations
in pregnant women are associated with increased fetal loss and low
birth weight
(November 2004)
The background of the study. The extent to which
maternal hyperthyroidism affects pregnancy is uncertain. This study
evaluated the effects of the high serum thyroxine (T4) and triiodothyronine
(T3) concentrations in women and men with the syndrome of resistance
to thyroid hormone on the course of pregnancy and on the development
and thyroid function of their offspring.
How the study was done. The study subjects were
167 members of a large family with resistance to thyroid hormone
caused by a mutation in the T3 nuclear receptor. Among them, 44
carried the mutation, and 123 did not. The subjects were identified
by biochemical findings, and the diagnosis was confirmed by mutation
analysis. The subjects with the mutation had high serum free T4
index and free T3 index values and normal serum thyrotropin (TSH)
concentrations; the respective values were normal in all the other
subjects.
The results of the study. There were 18 couples
in which one subject carried the mutation and 18 couples of unaffected
first-degree relatives. The frequency of miscarriage was considerably
higher (24 percent) in the couples in which the woman carried the
mutation, as compared with the couples in which the man carried
the mutation (7 percent), the couples of unaffected first-degree
relatives (9 percent), and couples of unrelated subjects (8 percent).
The affected mothers delivered 20 affected infants and 11 unaffected
infants, and the spouses of the affected men delivered 15 affected
infants and 12 unaffected infants. The birth weight of the unaffected
infants of affected mothers was significantly lower than that of
the affected infants of affected mothers. Newborn screening blood-spot
thyrotropin (TSH) values were undetectable in 3 unaffected infants
of affected mothers, but similar (approximately 5 mU/L) in 8 affected
infants of affected mothers, 13 affected or unaffected infants of
affected fathers, and 19 infants of the unaffected first-degree
relatives.
The conclusions of the study. The miscarriage
rate is high among women with high serum thyroid hormone concentrations
caused by generalized thyroid hormone resistance. Their affected
infants have normal birth weight and normal serum TSH values, but
their unaffected infants have low birth weight and low serum TSH
values.
The original article. Anselmo J, Cao D, Karrison
T, Weiss RE, Refetoff S. Fetal loss associated with excess thyroid
hormone exposure. JAMA 2004;292:691-5.
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