Question

A 27-year-old woman presents to her primary care physician because she has not menstruated for the past $6$ months. She achieved menarche at age 14, and her menses have historically been regular, with bleeding lasting 4 days every 29 days. She works as a software engineer and exercises moderately twice a week. She recently started a new relationship and uses condoms for birth control. She has not noticed any breast discharge, changes in voice, or abnormal hair growth. Her body mass index is $24 \text{ kg/m}^2$. Her vital signs are normal. Physical examination shows normal thyroid size and no skin findings suggestive of hyperandrogenism. A urine pregnancy test performed at the clinic is negative. Which of the following is the most appropriate next step in the evaluation of this patient?