Management of TSH 5 in Pregnancy
For pregnant women with TSH of 5 mIU/L, initiate levothyroxine therapy immediately to prevent adverse maternal and fetal outcomes, including preeclampsia, low birth weight, and impaired fetal neurodevelopment. 1, 2, 3
Rationale for Immediate Treatment
TSH >2.5 mIU/L in the first trimester is abnormal during pregnancy and requires treatment, as pregnancy-specific reference ranges differ substantially from non-pregnant values 2, 3
Women with TSH levels between 2.5 and 5.0 mIU/L during pregnancy have a significantly increased pregnancy loss rate (6.1% vs 3.6% in women with TSH <2.5 mIU/L, P = 0.006), even when thyroid antibodies are negative 2
Untreated or inadequately treated hypothyroidism during pregnancy is associated with preeclampsia, low birth weight, and potential neurodevelopmental effects in the offspring 4, 1, 5, 6
Maternal hypothyroidism can impair fetal neural development, particularly during the first trimester when the fetus is entirely dependent on maternal thyroid hormone 6, 7, 3
Initial Levothyroxine Dosing
For new-onset hypothyroidism with TSH <10 mIU/L during pregnancy, start levothyroxine at 1.0 mcg/kg/day 8
For a typical 70 kg pregnant woman, this translates to approximately 75 mcg daily as the starting dose 8
If the patient has pre-existing hypothyroidism and is already on levothyroxine, increase the current dose by 12.5 to 25 mcg per day immediately 8
Monitoring Schedule
Check TSH and free T4 every 4 weeks after initiating or adjusting levothyroxine until TSH is within the trimester-specific normal range 8, 3
The target is to maintain TSH in the trimester-specific reference range, with first trimester upper limit of 2.5 mIU/L 2, 3
Continue monitoring thyroid function at minimum once each trimester throughout pregnancy 1, 5
Aim to maintain free T4 in the high-normal range using the lowest effective levothyroxine dose 1
Dose Adjustments During Pregnancy
Up to 75% of women with hypothyroidism require higher levothyroxine doses during pregnancy compared to pre-pregnancy levels 5
TSH elevations can appear as early as 4 to 8 weeks of gestation or as late as the third trimester 5
The mean levothyroxine dose needed during pregnancy is approximately 150 mcg/day, though individual requirements vary widely 5
Pregnancy-related dose increases typically range from 25-50% above pre-pregnancy doses 4
Postpartum Management
Immediately reduce levothyroxine to pre-pregnancy dosage after delivery 8
Recheck TSH 4 to 8 weeks postpartum to confirm appropriate dosing 8
Women with autoimmune thyroid disease may experience postpartum thyroiditis flares, requiring continued monitoring at 6 to 12 weeks postpartum 5, 7
Critical Pitfalls to Avoid
Never delay treatment waiting for repeat testing in pregnancy—the risks of untreated hypothyroidism to fetal neurodevelopment outweigh concerns about transient TSH elevation 6, 3
Do not use non-pregnant TSH reference ranges (0.45-4.5 mIU/L) during pregnancy, as the first trimester upper limit should be 2.5 mIU/L 2, 3
Avoid inadequate monitoring—failure to check thyroid function each trimester can result in undetected hypothyroidism and adverse outcomes 1, 5
Do not forget to reduce levothyroxine dose immediately postpartum, as continuing pregnancy doses increases risk of iatrogenic hyperthyroidism 8, 5