A 40-year-old woman comes to the office for evaluation of significant fatigue, weight gain, and constipation over the past two months. She was diagnosed with hypothyroidism 5 years ago and has been taking a stable dose of levothyroxine with good adherence. Her last TSH level 6 months ago was within normal limits. She has not experienced any recent stressors or illnesses, and her medical history is otherwise unremarkable. She started taking an oral contraceptive pill 4 months ago. She does not drink alcohol, smoke cigarettes, or use illicit drugs. Her body mass index is 30 kg/m2, blood pressure is 130/80 mm Hg, and pulse is 72/min. On physical examination, the thyroid gland is nonpalpable, and there are no other significant findings on examination.
Laboratory results are as follows:
Which of the following is the most likely cause of her current symptoms?
The correct answer is:
D) Increased thyroxine-binding globulin concentration
This patient with hypothyroidism has experienced an increase in her TSH levels and a recurrence of hypothyroid symptoms after starting oral contraceptive pills. Estrogen in oral contraceptives increases the concentration of thyroxine-binding globulin (TBG). Increased TBG levels lead to more thyroid hormone being bound and less free hormone available for metabolic activities, which can cause hypothyroid symptoms and increased TSH levels in patients on thyroid hormone replacement therapy. Patients with hypothyroidism on levothyroxine therapy require an increased dose of levothyroxine when starting estrogen therapy to compensate for the increased TBG levels.
Answer choice A: Decreased absorption of levothyroxine, is incorrect. Decreased absorption of levothyroxine can occur with certain medications (e.g., calcium, iron), but the patient has not reported taking such medications.
Answer choice B: Decreased levothyroxine dose requirement, is incorrect. Decreased levothyroxine dose requirement would not lead to increased TSH and hypothyroid symptoms.
Answer choice C: Increased hepatic metabolism of thyroid hormone, is incorrect. Increased hepatic metabolism of thyroid hormone can occur with drugs like rifampin or phenytoin but not with estrogen.
Answer choice E: Interference of oral contraceptive with levothyroxine action, is incorrect. Interference of oral contraceptive with levothyroxine action is not a recognized mechanism affecting thyroid hormone levels.
Key Learning Point
In patients with hypothyroidism taking levothyroxine, starting oral contraceptive pills can increase thyroxine-binding globulin (TBG) levels due to estrogen, requiring an increased dose of levothyroxine to maintain euthyroid status.
