Increased pregnancy loss rate in thyroid antibody negative women with TSH levels between 2.5 and 5.0 in the first trimester of pregnancy.

CONTEXT

The definition of what constitutes a normal TSH during pregnancy is in flux. Recent studies suggested that the first trimester upper limit of normal for TSH should be 2.5 mIU/liter.

OBJECTIVE

The objective of the study was to evaluate the pregnancy loss and preterm delivery rate in first-trimester thyroid peroxidase antibody-negative women with TSH values between 2.5 and 5.0 mIU/liter.

DESIGN

The present study is a component of a recently published large-scale prospective trial that evaluated the impact of levothyroxine treatment on maternal and neonatal complications in thyroid peroxidase-positive women with TSH levels above 2.5 mIU/liter. The present study evaluated 4123 thyroid peroxidase antibody-negative women with TSH levels at or below 5.0 mIU/liter. Women were divided into two groups based on their initial TSH: group A, TSH level below 2.5 mIU/liter, excluding hyperthyroid women defined as an undetectable TSH with an elevated free T(4), and group B, TSH level between 2.5 and 5.0 mIU/liter.

SETTING

The study was conducted at two ambulatory clinics of community hospitals in southern Italy.

PATIENTS

A total of 4123 women were evaluated.

INTERVENTION

There was no intervention.

MAIN OUTCOME MEASURES

The incidence of pregnancy loss and preterm delivery in group A as compared with group B was measured.

RESULTS

The rate of pregnancy loss was significantly higher in group B as compared with group A (6.1 vs. 3.6% respectively, P = 0.006). There was no difference in the rate of preterm delivery between the two groups.

CONCLUSIONS

The increased incidence of pregnancy loss in pregnant women with TSH levels between 2.5 and 5.0 mIU/liter provides strong physiological evidence to support redefining the TSH upper limit of normal in the first trimester to 2.5 mIU/liter.