Management of Subclinical Hypothyroidism in Pregnancy with TSH 4.8
Pregnant women with subclinical hypothyroidism and a TSH of 4.8 should be treated with levothyroxine to restore serum TSH to the reference range, with treatment targets of ≤2.5 mIU/L for the first trimester and ≤3 mIU/L for the second and third trimesters. 1
Rationale for Treatment
- Untreated maternal hypothyroidism, even subclinical, increases risks of adverse pregnancy outcomes including preeclampsia, low birth weight, fetal wastage, and potential neuropsychological complications in offspring 1
- The potential benefit-risk ratio of levothyroxine therapy justifies its use in pregnant women with subclinical hypothyroidism 2
- Recent evidence shows that levothyroxine therapy that controls serum TSH levels throughout pregnancy can reduce the risks of pregnancy complications 3
Initial Dosing Recommendations
- For subclinical hypothyroidism with TSH between 2.5-4.2 mIU/L, start with 1.20 μg/kg/day of levothyroxine 4
- For TSH between 4.2-10 mIU/L (which includes our patient with TSH 4.8), start with 1.42 μg/kg/day of levothyroxine 4
- Alternatively, a fixed starting dose of 50-75 μg/day can be used for TSH between 2.5-8.0 mIU/L 5
Monitoring and Dose Adjustments
- Check TSH and free T4 levels every 4-6 weeks during pregnancy 6
- The first dose adjustment should be made as early as possible, ideally within the first trimester 6
- Many patients will require additional adjustments during the second and third trimesters 6, 5
- The requirement for levothyroxine often increases during pregnancy due to increased metabolic demands 1, 6
Expected Treatment Response
- With appropriate initial dosing, approximately 80-90% of women with subclinical hypothyroidism will achieve target TSH levels without requiring dose adjustments 4
- Euthyroidism can typically be achieved within approximately 6 weeks of starting treatment 4
- After delivery, the levothyroxine dose should return to the pre-pregnancy dose 7
Special Considerations
- Certain foods and medications can affect levothyroxine absorption and should be taken at least 4 hours apart from levothyroxine 7
- Soybean flour, cottonseed meal, walnuts, dietary fiber, and grapefruit juice may decrease levothyroxine absorption 7
- Concurrent use of certain medications (oral anticoagulants, digitalis glycosides, antidepressants) may require dose adjustments of those medications 7
Potential Complications of Untreated Subclinical Hypothyroidism
- Increased risk of hypertensive disorders of pregnancy 3
- Higher incidence of premature rupture of membranes 3
- Increased risk of neonatal complications including cardiac defects, hyperbilirubinemia, and pneumonia 3
By promptly initiating levothyroxine treatment for this pregnant woman with a TSH of 4.8, the risks of maternal and fetal complications can be significantly reduced while maintaining an excellent safety profile.