Final answer:
Free thyroid hormone levels usually stay normal during pregnancy and when using oral contraceptives due to the body's regulatory mechanisms. Pregnancy increases thyroid hormone production.
Step-by-step explanation:
The levels of free thyroid hormone are typically maintained within a normal range during pregnancy and when using oral contraceptives. During pregnancy, the production of thyroid hormone is increased to raise the maternal metabolic rate due to the increased levels of thyrotropin, a pituitary hormone, and placental hormones. It is this hormonal balance that ensures the mother's body can cope with the demanding changes of pregnancy, such as increased appetite and hot flashes.
In contrast, oral contraceptive use affects hormone levels by providing a constant level of estrogen and progesterone. These hormones negatively feed back onto the hypothalamus and pituitary, preventing the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), effectively suppressing ovulation.
However, despite these changes, the mechanisms in the body work to regulate the levels of thyroxine (T4) and triiodothyronine (T3) so that free T3 and T4 levels remain adequate for the body's needs, therefore typically remaining in the normal range during these two different conditions.