Final answer:
Free thyroid hormone levels usually remain within the normal range during pregnancy and oral contraceptive use despite hormonal changes that can influence thyroid function.
Step-by-step explanation:
The levels of free thyroid hormone generally remain normal during pregnancy and with the use of oral contraceptives even though there are physiological changes that affect thyroid function. During pregnancy, hormones such as thyrotropin and placental hormones increase thyroid hormone production which elevates the maternal metabolic rate and supports the growing fetus. Meanwhile, oral contraceptives provide a constant level of estrogen and progesterone, which via negative feedback, prevent the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), thus influencing the menstrual cycle without significantly affecting free thyroid hormone levels. It is important to remember that thyroid function tests may be influenced by these factors, and the normal range for thyroid hormones may have different reference values during pregnancy. These tests typically measure free T3 and T4 levels, which are the unbound, biologically active forms. These levels are kept within a normal range thanks to the regulatory feedback system involving TRH (thyrotropin-releasing hormone), TSH (thyroid-stimulating hormone), and T4 (thyroxine).