Hypothyroid Symptoms Throughout Pregnancy Are Predominantly Associated with Thyroxine and Not with Thyrotropin Concentrations

Background: It is unclear whether levels of hypothyroid symptoms in pregnant women with (sub)clinical thyroid dysfunction differ from euthyroid controls and whether free thyroxine (fT4)/thyrotropin (TSH) changes throughout pregnancy affect hypothyroid symptom levels. The objective was twofold: (1) To compare hypothyroid symptom levels between thyroid dysfunction subgroups and a carefully defined reference group; (2) to assess the association between fT4/TSH changes throughout pregnancy and hypothyroid symptom levels adjusted for depressive symptoms. Methods: The current study was a longitudinal prospective cohort study in 1800 healthy pregnant women. At each trimester of pregnancy, hypothyroid symptoms were assessed with a 12-item symptom hypothyroidism checklist and depressive symptoms with the Edinburgh Depression Scale. Thyroid dysfunction was defined using the 2.5–97.5th fT4/TSH percentile of thyroid peroxidase antibodies-negative women. Euthyroid controls consisted of women with appropriate fT4 levels within the 10–90th percentile and with a normal TSH level. Hypothyroid symptom mean scores were compared between controls and several thyroid dysfunction subgroups. Growth mixture modeling was performed to evaluate possible longitudinal trajectories of hypothyroid and depressive symptoms. The association between hypothyroid symptom trajectories (adjusted for depression) and fT4/TSH changes was assessed with multivariate logistic regression analysis. Results: Women with overt hypothyroidism (fT4 < 2.5th, TSH >97.5th) and hypothyroxinemia (fT4 < 2.5th, TSH: 2.5–97.5th) showed higher hypothyroid symptom levels compared with the euthyroid controls and women with subclinical hypothyroidism (SCH, fT4: 2.5–97.5th, TSH >97.5th), because 82% of these SCH women had fT4 levels in the euthyroid range. Two groups of hypothyroid and depressive symptoms were defined: a persistently low and persistently high symptom group. fT4 decreased in 98% of the women from the first to third trimester and per unit pmol/L fT4 decrease (not TSH increase), the likelihood to present persistently high hypothyroid symptoms increased with 46%, adjusted for depression. Conclusions: A properly defined euthyroid control group distinguishes women with hypothyroid symptoms. An fT4 decrease toward end term is associated with persistently high hypothyroid symptom levels. Clinicians should be aware of the importance of fT4 stratification in SCH women.