We measured TSH, free T3, free T4, TPO-Ab and Tg-Ab levels for a sample population resident in the Aragon region of Spain consisting of 1198 pregnant women representing different ethnic groups.
For the 1021 pregnant women with normal TPO-Ab and Tg-Ab levels, TSH and free T4 values were different depending on the pregnancy trimester, with statistically significant differences in these values for the 3 trimesters (P < .001), but not in free T3 values. It is important to have reference values for the 3 trimesters, particularly for TSH and free T4, and particularly for the first trimester, given that maternal thyroid hormone dysfunction may affect optimal development of the fetus [13, 14].
Our results are different from those for other studies. Kurioka et al  reported significantly reduced levels of free T3 and free T4 and raised values of TSH as pregnancy progressed; and Kumar et al  reported TSH values that increased steadily with each trimester, and free T3 and free T4 values that increased in the first and second trimester but decreased in the last trimester. Our results are similar, however, to those reported by Marwaha et al , with the exception of the free T4 values, which, according to these authors, decreased as gestational age advanced. It was further observed from our data that only TSH showed significant differences (P < .005) according to the age of the mother.
Considering the full 9 months of pregnancy, and despite the fact that there was no evidence of the presence of thyroid antibodies or thyroid diseases, we observed 3 cases of high free T3 values (5.01 pg/mL, 6.09 pg/mL, and 7.1 pg/mL). For the mother with a free T3 value of 7.1 pg/mL, free T4 and TSH values were 2.13 ng/dL and 0.0018 μUI/mL, respectively. As for free T4, 2 first-trimester pregnant women with no evident pathologies had values of 1.63 ng/dL and 1.75 ng/dL, which, however, returned to normal levels in the second trimester.
Considering the entire gestational period, we observed 4 cases with free T4 levels above the manufacturer's upper limit (1.76 ng/dL, 1.75 ng/dL, 1.63 ng/dL, and 1.59 ng/dL), accompanied by TSH values below the lower limit. These may be cases of where physiological changes affected thyroid gland functioning and interpretation of the thyroid function tests [7, 18, 19].
Analyzing TSH data, the mean for our population was below the manufacturer's lower limit (<2.5 μUI/mL), and there were 5 cases with values above the upper limit (4.17 μUI/mL, 4.30 μUI/mL, 4.33 μUI/mL, 4.48 μUI/mL, and 5.17 μUI/mL); this percentage (0.59%) is lower than that reported for other studies .
The pathologies present in the women without thyroid antibody disorders were as follows (number of cases in brackets): hypothyroidism that resolved with treatment (6); hypothyroidism with gestational diabetes and low but normal TSH (1); thyroidectomy (1); goiter (2), multinodular goiter (1), cold nodule (1); and gestational diabetes (5).
We found that 177 (14.77%) of the pregnant women were positive for TPO-Ab and Tg-Ab, indicating a level of autoimmune diseases of the thyroid in our population that is very similar to that found in other studies [21–23].
The mean TSH in our population was found to be below the manufacturer's lower limit (<2.5 μUI/mL); nonetheless, similarly to findings in studies by other authors, we observed that high TPO-Ab and Tg-Ab values were associated with an increase in TSH values .
Finally, women with high levels of thyroid antibodies had medical histories as follows (number of cases in brackets): hypothyroidism (5, of which 2 had a previous history of miscarriage); thyroidectomy (2); simple goiter (1); chronic autoimmune thyroiditis (1); hyperthyroidism (1), thyroidectomy (2); fetal death (3); ectopic pregnancy (1); gestational diabetes (3); breast fibroadenomas (6), miscarriage (23); and repeated miscarriage (7).
In conclusion, of the 1198 pregnant women resident in the Aragon region of Spain analyzed in our study, 85.22% had normal TPO-Ab and Tg-Ab values and 14.77% tested positive for autoimmune diseases of the thyroid. In regard to the population with antibody values that were normal for the gestational stage, free T4 and TSH values, but not free T3 values, were statistically different.
The data from this study may be useful in establishing reference values for our population, in establishing limits in regard to detecting thyroid diseases such as Graves disease or Hashimoto thyroiditis, and in evaluating risk to the fetus or neonate as a result of maternal thyroid dysfunction.