Preconception Thyrotropin Levels and Thyroid Function at Early Gestation in Women With Hashimoto Thyroiditis

Area of Science:

• Endocrinology
• Reproductive Medicine
• Thyroidology

Background:

• Preconception optimization of thyroid function is recommended for women with Hashimoto thyroiditis (HT) to mitigate risks of thyroid insufficiency during early gestation.
• Early gestation thyroid insufficiency can arise even in women with preconception thyrotropin (TSH) levels ≤ 2.5 mIU/L.

Purpose of the Study:

• To evaluate the prevalence of first-trimester thyroid insufficiency in HT women with preconception TSH ≤ 2.5 mIU/L, with or without levothyroxine (LT4) treatment.
• To establish preconception TSH cutoffs that accurately predict the need for LT4 adjustment or prescription in the first trimester.

Main Methods:

• Prospective follow-up of 260 HT pregnant women (122 on LT4 [Hypo-HT], 138 euthyroid without LT4 [Eu-HT]) from preconception to term.
• Analysis of serum TSH at 4-6 week intervals.
• Use of Receiver Operating Characteristic (ROC) curves to determine TSH cutoffs for predicting first-trimester TSH levels > 2.5 mIU/L and > 4.0 mIU/L.

Main Results:

• Approximately 30% of both Hypo-HT and Eu-HT women had TSH > 2.5 mIU/L in the first trimester.
• First-trimester TSH > 4.0 mIU/L was observed in 19.7% of Hypo-HT and 10.1% of Eu-HT women.
• Optimal preconception TSH cutoffs were identified: 1.24/1.74 mIU/L for Hypo-HT and 1.73/2.07 mIU/L for Eu-HT women, predicting TSH > 2.5 and > 4.0 mIU/L, respectively. Exceeding these cutoffs significantly increased the risk of thyroid insufficiency.

Conclusions:

• Established preconception TSH cutoffs can identify HT women at high risk for first-trimester thyroid insufficiency.
• These cutoffs are lower than target gestational TSH levels, suggesting a need for proactive management.
• Implementing these preconception TSH thresholds can guide timely LT4 adjustments, improving pregnancy outcomes for women with HT.