USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

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Which condition is characterized by an increase in total serum T4 without affecting free T4 and TSH levels due to changes in TBG?

Estrogen Effects on TBG and Thyroid Hormones
Pituitary apoplexy
Subacute granulomatous thyroiditis
↓Serum TSH

The correct answer is: Estrogen Effects on TBG and Thyroid Hormones

The condition characterized by an increase in total serum T4 without affecting free T4 and TSH levels is indeed related to estrogen effects on thyroxine-binding globulin (TBG) and thyroid hormones. Elevated levels of estrogen, such as during pregnancy or from estrogen replacement therapy, can increase the production of TBG. This, in turn, leads to an increase in the total serum T4 levels since T4 is bound to TBG. However, this does not change the levels of free T4 or TSH because the quantity of thyroid hormone that is bioavailable (free T4) remains unchanged. The measurement of total T4 reflects both free T4 and the hormone bound to TBG, which explains the observed increase with rising TBG levels. In this scenario, the normal free T4 and TSH levels indicate that the thyroid function remains unaffected, distinguishing this condition from others where there would be aberrations in these hormones. Other listed conditions involve different pathological processes that would typically lead to alterations in free T4 or TSH levels, marking them as distinct from this specific hormone interaction.