Reference Levels for TSH in Iodine-Sufficient Low-Risk Pregnant Women

Recent evidence suggests that thyrotropin (TSH) levels are population specific because of differences in ethnicity. As a result, the 2017 ATA guidelines state that treatment may be tailored as per the laboratory-specific reference ranges of TSH for the local population instead of using a fixed upper limit of 2.5 mIU/L during pregnancy.

This was a cross-sectional study in which we collected detailed clinical data of 604 pregnant women along with their TSH and spot urinary iodine excretion levels. Reflex testing for thyroid peroxidase antibodies (TPOAb) was done in women with TSH levels > 2.5 mIU/L in 1st trimester and 3.0 mIU/L in 2nd and 3rd trimester. After excluding 295 women who had high risk factors as per ATA 2017 guidelines and those who were TPOAb positive, we calculated the reference range for TSH in an iodine-sufficient low-risk cohort of 309 women.

With median urinary Iodine of 255 µg/l, our population had more than required iodine levels. The 5th and 95th percentiles of TSH in our study cohort of 604 women were 0.64 and 7.81 mIU/L, respectively, while the 5th and 95th percentiles of TSH for the low-risk cohort of 309 women were 0.59 and 4.48 mIU/L, respectively.

An upper limit of 4.5 mIU/L for TSH level during pregnancy can be used to guide management decisions for low-risk North Indian women.