Effects of Levothyroxine on Pregnant Women With Subclinical Hypothyroidism, Negative for Thyroid Peroxidase Antibodies.

Context: Currently, there is no consensus on universal thyroid screening and levothyroxine (LT4) treatment of pregnant women with subclinical hypothyroidism (SCH) who are negative for thyroid peroxidase antibody (TPOAb-).

Objective: We aimed to evaluate the benefits of LT4 treatment on pregnancy outcomes in SCH-TPOAb- women.

Design: This study was conducted within the framework of the Tehran Thyroid and Pregnancy Study. A single-blind randomized clinical trial was undertaken in pregnant women who were SCH-TPOAb-.

Setting: Prenatal care centers of the Shahid Beheshti University of Medical Sciences.

Patients: Using the thyrotropin (TSH) cut point of 2.5 mIU/L, 366 SCH-TPOAb- and 1092 euthyroid TPOAb- women were recruited.

Intervention: SCH-TPOAb- women were randomly assigned to two groups: group A (n = 183) who were treated with LT4 and group B (n = 183) who received no treatment. A total of 1,028 euthyroid TPOAb- women served as the control group (group C).

Main Outcome Measure: The primary outcome was the rate of preterm delivery.

Results: Using the TSH cutoff of 2.5 mIU/L, no significant difference in preterm delivery was observed between groups A and B [relative risk (RR): 0.86; 95% confidence interval (CI): 0.47 to 1.55; P = 0.61]. However, log-binomial model analysis based on a cut point of 4.0 mIU/L demonstrated a significantly lower rate of preterm delivery in LT4-treated women compared with those who received no treatment (RR: 0.38; 95% CI: 0.15 to 0.98; P = 0.04).

Conclusions: Despite no beneficial effect of LT4 therapy in reducing preterm delivery in SCH-TPOAb- women with a TSH cut point of 2.5 to 4 mIU/L, LT4 could precisely decrease this complication using the newly recommended cutoff ≥4.0 mIU/L.