Establishment of Trimester-Specific Thyroid Reference Intervals in Singleton and Twin Pregnancies

Every pregnancy is a unique biological journey, but for the developing baby, the mother's thyroid gland is a constant, vital partner. This small, butterfly-shaped organ in the neck produces hormones that act as a master regulator for growth and brain development. In the early weeks of pregnancy, before the baby's own thyroid gland is fully functional, the fetus relies entirely on the mother's supply. As the pregnancy progresses, the mother's body undergoes a massive physiological shift to support two lives. Her blood volume increases, her kidneys filter differently, and a surge of pregnancy hormones alters how her thyroid hormones travel through her bloodstream. Because of these changes, the "normal" levels of thyroid hormones in a pregnant woman are different from those in a non-pregnant woman, and they shift again as the pregnancy moves from the first to the third trimester. If doctors use standard, non-pregnant reference ranges to check a pregnant woman's thyroid, they might mistakenly diagnose a healthy woman with a disorder or miss a real problem, potentially risking the baby's development.

For decades, medical guidelines have largely relied on data from singleton pregnancies—those carrying one baby—to set these safety ranges. However, a twin pregnancy is a different biological event. The increased placental mass in twin pregnancies means a much larger surge of pregnancy hormones and a greater demand on the mother's system. Until now, it has been unclear whether the thyroid hormone levels in women carrying twins follow the same patterns as those carrying one baby, or if they require their own specific guidelines. A team of researchers at West China Second University Hospital set out to answer this question by looking directly at the blood of pregnant women to see how their thyroid hormones behaved across the three stages of pregnancy.

The researchers conducted a detailed review of medical records from women who received prenatal care at their hospital between January 2023 and March 2025. They focused on 1,169 women who had healthy pregnancies without pre-existing thyroid issues, dividing them into two groups: 728 women carrying one baby and 441 women carrying twins. For each woman, the team collected blood samples during the first, second, and third trimesters to measure five key thyroid indicators: total T3, total T4, thyroid-stimulating hormone (TSH), free T3, and free T4. By analyzing these samples, the researchers established specific reference intervals for each group. These intervals represent the range of values found in the middle 95 percent of healthy women, serving as a benchmark to identify who might need medical attention.

The study revealed that the thyroid hormone profiles of women carrying twins are distinct from those carrying one baby, and these differences change depending on how far along the pregnancy is. In the first trimester, women with twins showed higher levels of total T3 and TSH, but lower levels of free T4 compared to women with singletons. As the pregnancy moved into the second trimester, the women with twins continued to show higher levels of total T3 and free T3. By the third trimester, the difference became even more pronounced, with the twin group displaying significantly higher levels of total T3, total T4, TSH, and free T3 than the singleton group. The researchers noted that the range of values for TSH in twin pregnancies was particularly wide, suggesting that the thyroid response in these pregnancies is more variable and complex.

These findings suggest that the standard reference ranges used for most pregnancies may not be accurate for women carrying twins. If a doctor were to apply the guidelines designed for a singleton pregnancy to a woman carrying twins, the natural, healthy variations in her hormone levels could be misinterpreted as a disorder. For instance, a TSH level that is perfectly normal for a woman with twins might appear dangerously high if judged against a singleton standard. The researchers concluded that establishing separate, trimester-specific reference intervals for twin pregnancies is essential. By creating these tailored guidelines, medical professionals can better interpret thyroid function tests, ensuring that women with twins receive accurate diagnoses and appropriate care without the risk of unnecessary treatment or missed health issues.