Question of the Week
For December 24, 2019
A 53-year-old woman has been treated with 100 µg of levothyroxine daily since being diagnosed with Hashimoto thyroiditis at 32 years of age. Her thyroid-stimulating hormone levels have ranged from 1.2 to 2.6 µU/mL (reference range, 0.5–4.7).
She reports some fatigue that she believes is from hot flashes in the setting of menopause. She has a history of hypertension, controlled with the use of hydrochlorothiazide 12.5 mg daily. About 6 months ago, she became aware of vitamin recommendations for postmenopausal women and began taking a combined calcium–vitamin D supplement twice daily as well as a multivitamin every morning with her hydrochlorothiazide and levothyroxine.
Physical examination reveals a blood pressure of 124/81 mm Hg, a weight of 60 kg, and a height of 165 cm. The thyroid is atrophic, but the remainder of the examination is unremarkable. The patient’s thyroid-stimulating hormone level is now 16.8 µU/mL.
What is the most likely cause of this patient’s increase in thyroid-stimulating hormone level?