Endocrine Abstracts (2011) 26 S18.3

Thyroid autoimmunity in pregnancy

Kris Gustave Poppe

UZ Brussel - VUB, Brussels, Belgium.

To adapt the working of the thyroid to the changes occurring during pregnancy, an adequate availability of iodine and the integrity of the thyroid gland are needed. With regard to thyroid function assessment during pregnancy, is it important to be aware of the differences in accuracy of the different FT4 assays. Thyroid autoimmunity (TAI) is the most common cause of hypothyroidism before and during pregnancy and many women with TAI and a normal thyroid function in early gestation tend to become hypothyroid during gestation. In euthyroid women, with and without TAI, similar pregnancy rates have been reported, but many studies showed that the presence of TAI is associated with a significantly increased risk in the overall miscarriage rate, independently of the presence of other autoantibodies. The association between TAI and miscarriage does not imply a causal relationship, and underlying causes might be (a combination of) immune disorders, inappropriate TH levels for the pregnancy period, and an increased maternal age in women with TAI. Concerning intervention trials, Negro et al. showed that L-T4 in TAI + euthyroid pregnant women, reduced the miscarriage rate to that in women without TAI, a finding that was recently confirmed in an assited reproductive setting by an other group. The risk of spontaneous abortion during early gestation is also known to be increased in women with overt and subclinical hypothyroidism independently of the presence of TAI and in one study a positive linear relationship between fetal/child loss and serum TSH was shown. L-thyroxine is the drug of choice for the treatment of hypothyroidism and in women already receiving it, it should be adjusted before or during pregnancy to reach a serum TSH level ≤2.5 mIU/l during the first trimester. In women in whom hypothyroidism is diagnosed during pregnancy, L-T4 should immediately be initiated according to their body weight. The increase in the L-T4 dose becomes manifest after 4–8 weeks gestation and the magnitude of L-T4 changes depends on the etiology of the hypothyroidism. For all hypothyroid L-T4-treated women, serum TSH levels should be monitored every 6–8 weeks unless an increase in dosage is needed. After parturition, most hypothyroid women need to decrease the L-T4 dose received during pregnancy, and when TAI was associated, monitoring of thyroid function for at least 6 months after delivery is advised.

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