open access: Nature Reviews: Key Advances in Medicine

ENDOCRINOLOGY
in pregnancies of women who were negative
for TPO antibodies. 7 No association
with TPO-antibody positivity was observed
for 12 other adverse outcomes, including
preterm delivery between 34–37 weeks.
Although corrected for multiple comparisons,
the statistical tests applied were onetailed
rather than two-tailed. Given that the
association with respiratory distress is novel,
this finding could be a type 1 statis tical error.
The key feature of this study was that all the
par ticipants were chosen to have low-normal
TSH levels, and thus the results appear to
show that autoimmunity per se is responsible
for these adverse out comes. How ever,
miscarriage rates, pre viously shown to be
increased in TPO-antibody-positive women,
were not specifically increased and although
below 2.5 mU/l, TSH levels were significantly
higher in the TPO-antibody-positive women
than in control women. Although the results
clearly show that a trial of levothyroxine
during pregnancy is warranted in this subset
of TPO-antibody-positive women with lownormal
TSH levels, the pathogenic mechanism
behind the adverse outcomes of such
pregnancies is still in question.
Postpartum thyroiditis is a transient flaring
of pre-existing autoimmune thyroidi tis in
the year after delivery, which seems to be a
consequence of the restoration of nor mal
immune function after a period of altered
immuno regulation during pregnancy. 6
Tran sient abnormalities of thyroid dysfunction
are common in postpartum thyroiditis;
however, as has been clear for over two
decades, 2–21% of such patients have permanent
hypo thyroidism at the end of the year
after delivery, and this figure rises during
follow-up over subsequent years. A survey of
4,394 women from Southern Italy reported
in 2011 that 3.9% had postpartum thyroiditis
and 54% of these were hypo thyroid at 1 year
after delivery. 8 This figure for the prevalence
of post partum thyroiditis is rather low
compared with that in many other studies, a
difference that could be related to the exclusion
of women with any thyroid dysfunction
during the first trimester of pregnancy in the
2011 study. In addition, the women in this
Italian study were only screened twice postpartum,
so some cases of dysfunction might
have been missed. By contrast, the figure for
the prevalence of hypothyroidism at 1 year
is strikingly high. This finding might also be
related to the study design—if women with
milder forms of postpartum thyroiditis were
excluded because of limited sampling, this
exclusion could have distorted the overall
prevalence results, given that such women are
Key advances
■ Maternal thyroid function has a
measurable effect on fetal thyroid
hormone levels throughout pregnancy 1,3
■ Fetal thyroid hormone levels have an
important role in regulating fetal growth,
as measured by birth weight, length and
skin-fold thickness 3
■ Very preterm delivery (