From the Research
TSH levels typically decrease during the first trimester, reaching their lowest point around 8-10 weeks of gestation, and then gradually return to normal by approximately 16-20 weeks, while hCG levels peak around 8-10 weeks of gestation and then begin to decline, reaching a plateau by about 16-20 weeks. This pattern of change is due to the cross-reactivity of hCG with TSH receptors, which temporarily suppresses TSH production 1. The initial rise in hCG is crucial for maintaining the corpus luteum and progesterone production until the placenta takes over this function. Understanding these hormonal patterns is important for correctly interpreting thyroid function tests during pregnancy, as the temporary suppression of TSH is a normal physiological response rather than an indication of hyperthyroidism 2. Some studies have shown that hCG has thyrotropic activity, which, through rises in thyroid hormone levels, suppresses TSH secretion 3. It is also important to note that thyroid function tests provide information at physiological, pathological, and anatomical levels, and that serum TSH concentration is physiologically lower than the non-pregnant population 4. The most recent study on thyroid physiology in pregnancy recommends that method- and gestation-specific reference ranges be used for interpreting results in pregnancy 5. Key points to consider include:
- TSH levels decrease during the first trimester and return to normal by 16-20 weeks
- hCG levels peak around 8-10 weeks and then decline to a plateau by 16-20 weeks
- Cross-reactivity of hCG with TSH receptors suppresses TSH production
- Understanding hormonal patterns is crucial for interpreting thyroid function tests during pregnancy
- Thyroid function tests provide information at physiological, pathological, and anatomical levels
- Serum TSH concentration is physiologically lower than the non-pregnant population
- Method- and gestation-specific reference ranges should be used for interpreting results in pregnancy.