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

Which condition is associated with alterations in total serum T4 due to changes in thyroxine-binding globulin (TBG)?

1. Empty sella syndrome

2. Estrogen Effects on TBG and Thyroid Hormones

3. Hashimoto thyroiditis

4. Pituitary apoplexy

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

The correct choice indicates that alterations in total serum T4 levels can occur due to changes in thyroxine-binding globulin (TBG), which is influenced by estrogen. Estrogen increases the synthesis of TBG in the liver, leading to increased total T4 levels. This elevation in total T4 does not reflect a true increase in thyroid hormone production or an increase in free T4, which is the active form of the hormone. As a result, conditions that lead to elevated estrogen levels, such as pregnancy or estrogen therapy, can result in elevated total T4 due to increased TBG levels. In contrast, other conditions listed do not primarily involve changes in TBG. Hashimoto thyroiditis is an autoimmune condition that leads to hypothyroidism and would generally reduce levels of circulating thyroid hormones rather than affecting TBG. Empty sella syndrome and pituitary apoplexy are conditions affecting the pituitary gland and do not directly cause changes in TBG levels or resultant changes in total serum T4. Thus, the direct connection between estrogen, TBG, and total serum T4 makes the choice associated with estrogen effects on TBG and thyroid hormones the correct answer.