During an initial prenatal evaluation at weeks of gestation, a nulliparous woman reports experiencing daily nausea and intermittent vomiting over the past three weeks. She notes a -kg (-lb) weight loss since her positive pregnancy test but is able to tolerate small, frequent meals. Her medical history is unremarkable, and she takes a daily prenatal vitamin. Vital signs demonstrate a pulse of /min and a blood pressure of mm Hg. Physical examination reveals a normal, non-enlarged thyroid gland without palpable nodules. Cardiac auscultation shows a regular rhythm without murmurs.
Laboratory studies return the following results:
| Test | Result | Reference Range |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | U/mL | U/mL |
| Total thyroxine (T4) | g/dL | g/dL |
| Free thyroxine (Free T4) | ng/dL | ng/dL |
Which of the following mechanisms best explains this patient's specific combination of thyroid laboratory findings?
- Cross-reactivity of a placental glycoprotein with the thyrotropin receptor and augmented hepatic production of carrier globulinsCevap
- BAutoantibody activation of the thyrotropin receptor causing unregulated hormone release and suppression of pituitary feedback
- CDownregulation of the hypothalamic-pituitary-thyroid axis secondary to metabolic starvation from recurrent emesis
- DHyperproliferation of trophoblastic tissue leading to excessive thyroxine production independent of binding proteins
- EIncreased peripheral conversion of inactive thyroxine to active triiodothyronine due to elevated circulating cortisol
Cevap
Cross-reactivity of a placental glycoprotein with the thyrotropin receptor and augmented hepatic production of carrier globulins
The correct answer identifies the dual physiologic mechanisms of thyroid adaptation in early pregnancy. First, human chorionic gonadotropin (hCG)—a placental glycoprotein—shares a structurally identical alpha subunit with thyroid-stimulating hormone (TSH). At its peak around 10 weeks of gestation, hCG mildly stimulates the TSH receptor, causing a slight increase in thyroid hormone production that exerts negative feedback to suppress pituitary TSH secretion. Second, elevated estrogen levels stimulate hepatic synthesis of thyroxine-binding globulin (TBG) and decrease its clearance. The increased TBG binds more thyroxine, causing Total T4 levels to rise significantly, while the physiologically active Free T4 remains within the normal reference range.
Adım Adım Çözüm
Anahtar Kavram
Normal thyroid adaptations in early pregnancy include a transient beta-hCG-mediated suppression of TSH and an estrogen-mediated increase in thyroxine-binding globulin (TBG), resulting in elevated Total T4 but normal Free T4.