Thyroid Function during Pregnancy

Abstract

Background: This Case Conference reviews the normal changes in thyroid activity that occur during pregnancy and the proper use of laboratory tests for the diagnosis of thyroid dysfunction in the pregnant patient. Case: A woman in the 18th week of pregnancy presented with tachycardia, increased blood pressure, severe vomiting, increased total and free thyroid hormone concentrations, a thyroid-stimulating hormone (TSH) concentration within the reference interval, and an increased human chorionic gonadotropin (hCG) β-subunit concentration. Issues: During pregnancy, normal thyroid activity undergoes significant changes, including a two- to threefold increase in thyroxine-binding globulin concentrations, a 30–100% increase in total triiodothyronine and thyroxine concentrations, increased serum thyroglobulin, and increased renal iodide clearance. Furthermore, hCG has mild thyroid stimulating activity. Pregnancy produces an overall increase in thyroid activity, which allows the healthy individual to remain in a net euthyroid state. However, both hyper- and hypothyroidism can occur in pregnant patients. In addition, two pregnancy-specific conditions, hyperemesis gravidarum and gestational trophoblastic disease, can lead to clinical hyperthyroidism. The normal changes in thyroid activity and the association of pregnancy with conditions that can cause hyperthyroidism necessitates careful interpretation of thyroid function tests during pregnancy. Conclusion: Assessment of thyroid function during pregnancy should be done with a careful clinical evaluation of the patient’s symptoms as well as measurement of TSH and free, not total, thyroid hormones. Measurement of thyroid autoantibodies may also be useful in selected cases to detect maternal Graves disease or Hashimoto thyroiditis and to assess risk of fetal or neonatal consequences of maternal thyroid dysfunction.

Thyroid hormones states and brain development interactions

“…Evans et al (1999) demonstrated that maternal thyroid status regulates the ontogeny of monoaminergic and, to a lesser extent, cholinergic neurotransmitter metabolic enzyme activities in rat brain. These changes are evident during fetal life (Fantz et al, 1999) Fig. 8 Fig.…”

“…TBG, thyroid binding globulins; TTR, transthyretin; CSF, cerebrospinal fluid. Table 4 Thyroid function tests during pregnancy (Larsen et al, 1998;Fantz et al, 1999)…”

Ethnic differences in TSH but not in free T4 concentrations or TPO antibodies during pregnancy

“…1,2 While thyroid function and metabolism during pregnancy and the effects of thyroid dysfunction on pregnancy outcome have been documented, no data exist on ethnic differences in thyroid function during pregnancy. [1][2][3][4][5][6][7][8] The third National Health and Nutrition Examination Survey (NHANES III) showed that there are differences between blacks and whites in thyroid function in men and nonpregnant females. 9 Whites and Mexican Americans showed higher TSH concentration and thyroid peroxidase antibody (TPO-Ab) presence than blacks.…”

Thyroid function during pregnancy in women with past Graves' disease

“…birthweight, fetal loss and perinatal mortality. 3 However, we found normal thyroid function adaptation to pregnancy in women who had recovered from Graves' disease, as shown by negative tests for anti-TSH receptor antibodies and no treatment with anti-thyroid drugs and /or L-thyroxine. Therefore, the values for thyroid function parameters obtained in this study can be used as reference values for further studies.…”

“…1 Although free T3, free T4 and TSH levels usually remain within the normal range for non-pregnant women, a steady increase in thyroid T4 output has been reported. 2 In women with hypothyroidism due to thyroiditis, adaptation of thyroid function to pregnancy is inadequate, 3 and a 25% to 50% increase in the pre-pregnancy T4 dosage is needed to maintain normal serum thyrotropin concentrations during pregnancy. 4 The reason for this increase in T4 requirements is unclear.…”