Abstract
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. 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. 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. The study was conducted at two ambulatory clinics of community hospitals in southern Italy. A total of 4123 women were evaluated. There was no intervention. The incidence of pregnancy loss and preterm delivery in group A as compared with group B was measured. 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. 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.
Highlights
Context: The definition of what constitutes a normal TSH during pregnancy is in flux
A total of 84.4% of the women were in group A (n ϭ 3481) and 15.6% of women were in group B (n ϭ 642)
Per design of the study, the median TSH level in group A is significantly lower than group B (0.82 vs. 3.14 mIU/liter, Mann-Whitney U ϭ 0, P Ͻ 0.001)
Summary
Context: The definition of what constitutes a normal TSH during pregnancy is in flux. 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. The present study evaluated 4123 thyroid peroxidase antibody-negative women with TSH levels at or below 5.0 mIU/liter. Data on subclinical hypothyroidism (defined as a TSH Ն 5.0 mIU/liter) and miscarriage have been conflicting with some, but not all, studies showing an increased rate of pregnancy loss [8, 9] Both overt and subclinical hypothyroidism has been associated with preterm delivery [10]. The present study is the first to evaluate the relationship between spontaneous pregnancy loss, preterm delivery, and first-trimester TSH values between 2.5 and 5.0 mIU/liter. To remove this confounding factor, antibody-positive women have been excluded from the present investigation
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