Target TSH in Third Trimester of Pregnancy
The target TSH level in the third trimester of pregnancy is ≤3.0 mIU/L, with treatment recommended to maintain TSH within this range to prevent adverse maternal and fetal outcomes. 1, 2, 3
Trimester-Specific TSH Targets
The American Thyroid Association guidelines establish TSH targets of ≤2.5 mIU/L in the first trimester and ≤3.0 mIU/L in the second and third trimesters 4
These targets differ from non-pregnant reference ranges because pregnancy induces physiological changes in thyroid function, with TSH naturally declining in early pregnancy due to hCG stimulation of the thyroid gland 5
Research data from healthy pregnant women show TSH levels progressively increase through pregnancy, with mean values rising from 1.20 microIU/mL in the first trimester to 3.30 microIU/mL in the third trimester 6
Clinical Rationale for Treatment
Untreated or inadequately treated hypothyroidism during pregnancy increases risk of preeclampsia, low birth weight, and impaired fetal neuropsychological development 2, 3
Subclinical hypothyroidism (elevated TSH with normal free T4) during pregnancy warrants treatment with levothyroxine to restore TSH to the trimester-specific reference range 3
Even maternal hypothyroxinemia (low T4 with normal TSH) requires treatment to restore T4 levels to normal range, as this pattern is associated with alterations in fetal neuropsychological development and increased risk of fetal loss 2
Monitoring and Dose Adjustment
Thyroid function should be evaluated every trimester to adjust levothyroxine dose as necessary, with the goal of maintaining free T4 in the high-normal range using the lowest possible medication dose 2
For women with newly diagnosed hypothyroidism in the third trimester, the appropriate levothyroxine dose depends on the degree of TSH elevation: 1.20 μg/kg/day for TSH ≤4.2 mIU/L, 1.42 μg/kg/day for TSH >4.2-10 mIU/L, and 2.33 μg/kg/day for overt hypothyroidism 4
After initiating or adjusting levothyroxine, recheck TSH and free T4 in 4-6 weeks to ensure the target range is achieved 1
Important Caveats
Population-specific reference ranges may vary by geographic region and assay method, with some Chinese populations showing upper TSH limits of 5.40 mIU/L in the third trimester 7
However, the ATA guideline targets of ≤3.0 mIU/L for the third trimester remain the standard recommendation to minimize adverse pregnancy outcomes 4
Women with hypothyroidism planning pregnancy should ideally achieve a preconception TSH below 1.2 mIU/L, as levothyroxine requirements typically increase by 25-50% during pregnancy 3
Iodine deficiency during pregnancy can increase risk of congenital cretinism (mental retardation and neuropsychological defects), making adequate iodine intake essential 2