A 24-year-old woman comes to the clinic for a routine health maintenance examination. She … — USMLE Step 2 CK

A 24-year-old woman comes to the clinic for a routine health maintenance examination. She reports feeling well overall, though she notes occasional mild palpitations when she consumes excessive caffeine. She has no history of weight loss, heat intolerance, changes in bowel habits, or tremors. Four months ago, she was prescribed a combined oral contraceptive pill. Her only other medication is a daily multivitamin. Vital signs are within normal limits, with a pulse of $74/\text{min}$ and a blood pressure of $118/76\text{ mmHg}$ . Physical examination reveals a normal-sized thyroid gland without palpable nodules. Deep tendon reflexes are $2+$ bilaterally. Laboratory studies show the following:

Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$1.8\text{ \mu U/mL}$	$0.5 - 5.0\text{ \mu U/mL}$
Total thyroxine ( $T_4$ )	$14.8\text{ \mu g/dL}$	$5.0 - 12.0\text{ \mu g/dL}$
Free thyroxine ( $T_4$ )	$1.2\text{ ng/dL}$	$0.9 - 1.7\text{ ng/dL}$

Which of the following is the most appropriate next step in management?

Reassurance and continuation of current medicationsCevap
B
Initiation of low-dose methimazole therapy
C
Measurement of serum thyroid-stimulating immunoglobulins
D
Radioactive iodine uptake scanning
E
Discontinuation of the combined oral contraceptive pill

Cevap

Reassurance and continuation of current medications

The correct response is to provide reassurance and continue the current medications. The patient has a normal free T4 and normal TSH, confirming she is euthyroid. Over 99% of circulating thyroid hormone is bound to plasma proteins, primarily thyroxine-binding globulin (TBG). Estrogen, such as that found in combined oral contraceptive pills (as well as during pregnancy), decreases the clearance of TBG, leading to higher circulating TBG levels. This expands the pool of bound thyroid hormone, resulting in an elevated total T4 (and total T3). However, the hypothalamic-pituitary-thyroid axis quickly adjusts to maintain normal levels of biologically active free T4, which keeps TSH strictly within the normal range. Because this is a benign physiologic adaptation, no further diagnostic testing or medication adjustment is needed.

Adım Adım Çözüm

Evaluate the patient's clinical presentation and medication history.

The patient is asymptomatic (euthyroid) and is taking an estrogen-containing oral contraceptive.

Estrogen alters the synthesis and clearance of specific transport proteins in the liver, which can impact total hormone levels.

Analyze the thyroid laboratory panel.

Total T4 is elevated, but free T4 and TSH are completely normal.

It is critical to distinguish between true thyroid dysfunction (where free hormones and TSH are abnormal) and isolated alterations in binding proteins.

Synthesize findings to determine management.

The profile is consistent with estrogen-induced thyroxine-binding globulin (TBG) elevation.

Because the patient is euthyroid (normal free T4 and normal TSH), no medical intervention or medication change is necessary.

Anahtar Kavram

Estrogen (from oral contraceptives, hormone replacement therapy, or pregnancy) increases liver production and decreases clearance of thyroxine-binding globulin (TBG). This leads to an increase in total T4 and total T3 levels, but patients remain clinically euthyroid with normal free hormone levels and a normal TSH.

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