Treatment of Hypothyroidism in Pregnancy Based on TSH Levels
Pregnant women with elevated TSH levels should be treated with levothyroxine to restore the serum TSH concentration 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, 2
TSH Thresholds for Treatment
- Subclinical hypothyroidism in pregnancy is defined as TSH greater than the pregnancy-specific reference range for each laboratory, or TSH concentrations >2.5 mIU/L in the first trimester and >3 mIU/L in the second and third trimesters 2
- All pregnant women with overt hypothyroidism (typically TSH >10 mIU/L) should be treated immediately with levothyroxine 1
- Pregnant women with subclinical hypothyroidism (elevated TSH but below 10 mIU/L) should also receive treatment to normalize TSH levels 1
- Women planning pregnancy who have elevated TSH should be treated before conception to optimize thyroid function 1
Dosing Recommendations Based on Initial TSH Level
- For subclinical hypothyroidism with TSH >2.5-4.2 mIU/L: Initial levothyroxine dose of approximately 1.20 μg/kg/day 3
- For subclinical hypothyroidism with TSH >4.2-10 mIU/L: Initial levothyroxine dose of approximately 1.42 μg/kg/day 3
- For overt hypothyroidism with TSH >10 mIU/L: Initial levothyroxine dose of approximately 2.33 μg/kg/day 3
- A fixed dose of 50 mcg/day may be inadequate for many pregnant women with hypothyroidism, with only 42% achieving TSH <3 μU/mL on this dose 4
Monitoring and Dose Adjustments
- TSH levels should be monitored every 4 weeks until stable 1
- Once stable, TSH should be checked every trimester 1
- For women already on levothyroxine before pregnancy, increase the dose by approximately 30% when pregnancy is confirmed 5, 1
- Dose adjustments should aim to normalize TSH as quickly as possible to reduce risks to both mother and fetus 2, 3
- Monitor both TSH and free T4 levels, as some patients may have normal T4 but persistently elevated TSH or low T3 levels despite treatment 6
Rationale for Treatment
- Untreated maternal hypothyroidism increases risks of:
- Even subclinical hypothyroidism may be associated with adverse pregnancy outcomes, though evidence is less conclusive than for overt hypothyroidism 1
Special Considerations
- Women with autoimmune thyroid disease require careful monitoring as thyroid function can fluctuate during pregnancy 5
- Inadequate treatment of hypothyroidism is associated with poor outcomes, so achieving target TSH levels promptly is essential 3
- The requirement for levothyroxine often increases during pregnancy due to increased metabolic demands 1
By following these guidelines for TSH thresholds and appropriate levothyroxine dosing, the risks associated with maternal hypothyroidism can be minimized, improving outcomes for both mother and child.