FIRST TRIMESTER OF PREGNANCY REFERENCE RANGES FOR SERUM TSH AND THYROID TUMOR RECLASSIFIED AS BENIGN

Area of Science:

• Endocrinology
• Oncology
• Reproductive Medicine

Background:

• Current American Thyroid Association (ATA) guidelines use trimester-specific reference ranges for thyroid function interpretation during pregnancy.
• Recent studies indicate that first-trimester TSH levels fluctuate with gestational week, potentially challenging existing uniform ranges.
• Encapsulated follicular variant of papillary thyroid carcinoma (EFVPTC) is often managed similarly to classical papillary thyroid carcinoma despite its indolent behavior.

Purpose of the Study:

• To evaluate the variability of TSH levels within the first trimester of pregnancy based on gestational week.
• To propose a new nomenclature for noninvasive encapsulated follicular variant of papillary thyroid carcinoma based on its clinical behavior.

Main Methods:

• Analysis of a nationwide cohort of 6671 healthy pregnant Danish women to determine TSH variations.
• Retrospective reevaluation of 268 EFVPTC cases to assess outcomes and risk stratification.

Main Results:

• First-trimester TSH levels showed significant variation by gestational week, with levels up to week 6 aligning with nonpregnant ranges and a decrease noted between weeks 9-12.
• Noninvasive EFVPTC cases (Group 1) demonstrated a low risk of adverse outcomes, with patients remaining disease-free during 10-26 years of observation.
• These findings suggest that uniform TSH reference ranges for the entire first trimester may lead to misclassification and unnecessary treatment.

Conclusions:

• The interpretation of thyroid function in early pregnancy requires gestational week-specific TSH reference ranges for accurate assessment.
• A new classification, 'noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP)', is proposed for low-risk EFVPTC to reflect its indolent nature and guide management.