Randomised clinical trials have shown levothyroxine replacement may reduce the rates of preterm delivery in women with subclinical hypothyroidism and positive TPOAb.10 A single trial showed that levothyroxine treatment reduced the rate of miscarriage and pre-term delivery in euthyroid TPOAb positive women.11 This result has not been replicated in subsequent studies.10,12
There is mixed evidence to suggest that treatment of subclinical hypothyroidism in women requiring assisted reproductive technology improves delivery rates.13-15
Treatment of hypothyroidism detected in early pregnancy
In women with overt hypothyroidism (TSH level >10 mIU/L), levothyroxine should be commenced and referral to an endocrinologist should be considered.
In women with a TSH level of 2.5 to 10 mIU/L in early pregnancy, the recently updated American Thyroid Association guidelines recommend evaluating for TPOAb status.2 If TPOAb positive, treatment is recommended if the TSH level is 4.0 mIU/L or above (or above trimester-specific ranges). Treatment can be considered if the TSH level is 2.5 to 4.0 mIU/L. If TPOAb negative, treatment can also be considered if the TSH level is 4.0 mIU/L or above (or above trimester-specific ranges) (see Flowchart).
Once levothyroxine is commenced, thyroid function should be monitored four to six weekly until the TSH level is stable, then eight weekly with a final check at about 28 to 32 weeks’ gestation. Suggested starting levothyroxine doses are shown in the Table.
Following delivery, the dose of levothyroxine can be halved, or ceased if on 50 mcg daily or less during pregnancy, and thyroid function checked two to three months’ postpartum.
Treatment of pre-existing hypothyroidism
Most women with pre-existing hypothyroidism will need a 20 to 30% increase in the dose of levothyroxine when pregnancy is confirmed. In patients with no thyroid tissue left (e.g. congenital hypothyroidism, post total thyroidectomy or postradioactive iodine ablation) a dose increase of 50% may be required.
Thyroid function should be monitored four to six weekly until the TSH level is stable, then eight weekly with a final check at about 28 to 32 weeks’ gestation.
Following delivery, the levothyroxine dose can be returned to the prepregnancy dose, and thyroid function should be checked two to three months’ postpartum.
Untreated hyperthyroidism is also associated with a range of adverse pregnancy outcomes including low birthweight, preterm birth, pre-eclampsia and stillbirth. Hyperthyroidism should be appropriately managed before conception.16