Longitudinal trajectories of gestational thyroid function: A new approach to better understand changes in thyroid function

Context:  

Most studies of thyroid function changes during pregnancy use a cross-sectional design comparing means between groups rather than similarities within groups.

Objective:

Latent class growth analysis (LCGA) is a novel approach to investigate longitudinal changes that provide dynamic understanding of the relationship between thyroid status and advancing pregnancy.

Design: 

Prospective observational study with repeated assessments.

Setting: 

General community.

Patients: 

Eleven hundred healthy women were included at 12 weeks' gestation.

Main Outcome Measures: 

The existence of both free T4 (fT4) and TSH trajectories throughout pregnancy determined by LCGA.

Results: 

LCGA revealed three trajectory classes. Class 1 (n = 1019; 92.4%), a low increasing TSH reference group <had a gradual increase in TSH throughout gestation (from 1.1 to 1.3 IU/L). Class 2 (n = 30; 2.8%), a high increasing TSH grouP <displayed the largest increase in TSH (from 1.9 to 3.3 IU/L). Class 3 (n = 51; 4.6%), a decreasing TSH grouP <had the largest fall in TSH (from 3.2 to 2.4 IU/L). Subclinical hypothyroidism at 12 weeks occurred in up to 60% of class 3 women and was accompanied by elevated thyroid peroxidase antibodies (TPO-Ab) titers (50%) and a parental history of thyroid dysfunction (23%). In class 2, 70% of women were nulliparous compared with 46% in class 1 and 49% in class 3.

Conclusions:

LCGA revealed distinct trajectories of longitudinal changes in fT4 and TSH levels during pregnancy in 7.4% of women. These trajectories were correlated with parity and TPO-Ab status and followed patterns that might reflect differences in pregnancy-specific immune tolerance between nulliparous and multiparous women.