The thyroid produces 20% T3 and 80% T4, which is then metabolized into more T3 in the tissues. Thyroid hormones are crucial for almost every cell in our body, particularly for metabolism and cognitive function. Maintaining normal thyroid hormone levels during pregnancy is especially important for the baby’s cognitive development.
Pregnancy induces various physiological changes, including alterations in thyroid hormones, which means interpreting thyroid function tests and treating low thyroid function requires a slightly different approach.
Key Points to Consider:
1️⃣ B-HCG Effect:
- B-HCG, produced by the placenta (detected in pregnancy tests), stimulates the thyroid to produce more hormones.
- This effect peaks around 6-7 weeks, causing TSH levels to decrease.
- High B-HCG levels can lead to hyperthyroidism in some pregnant individuals, especially those with severe morning sickness or twins.
2️⃣ TSH as an Indicator:
- TSH is the most sensitive indicator of satisfactory thyroid levels, particularly during pregnancy.
- T3 and T4 are bound to thyroglobulin binding protein, which increases during pregnancy, reducing the amount of free hormone.
- Increased plasma volume in the latter part of pregnancy can dilute hormone levels.
- “Hypothyroxinaemia in pregnancy” refers to low T4 and normal TSH during the first trimester, which can negatively impact the baby.
3️⃣ T3 and the Placenta:
- T3 does not cross the placenta, and low T3 levels alone are not associated with adverse pregnancy outcomes.
- Mothers taking combined T3 and T4 or desiccated animal thyroid extract should switch to T4 before pregnancy, as only T4 can cross the placenta.
4️⃣ Baby’s Thyroid Function:
- After 20 weeks, the baby has its own thyroid and produces its own hormones.
- Mild hypothyroidism in the mother has less impact on the baby after this point, provided there is adequate iodine supply.