A 63-year-old woman comes to the physician because of hot flashes and heat intolerance for the last 2-3 months. The patient was taking conjugated estrogen therapy after having a total abdominal hysterectomy with bilateral salpingo-oophorectomy at age 50 for large fibroids and excessive uterine bleeding. However, she stopped taking estrogen 4 months ago after reading an article about the side effects of hormone replacement therapy. She has been treated for hypothyroidism for the past 15 years. Her thyroid function studies have been in the normal range, and her levothyroxine dose has not been changed in 8 years. She has no other medical issues and takes no other medications.
On examination, she appears to be slightly anxious. Her blood pressure is 140/66 mm Hg and pulse is 98/min. The thyroid gland is not palpable. She has mild lid lag but no proptosis. Fine hand tremors are present. There is no pretibial myxedema. The remainder of the physical examination is within normal limits.
Laboratory results are as follows:

Which of the following is the most likely cause of her thyrotoxicosis?

Question

A 63-year-old woman comes to the physician because of hot flashes and heat intolerance for the last 2-3 months.  The patient was taking conjugated estrogen therapy after having a total abdominal hysterectomy with bilateral salpingo-oophorectomy at age 50 for large fibroids and excessive uterine bleeding.  However, she stopped taking estrogen 4 months ago after reading an article about the side effects of hormone replacement therapy.  She has been treated for hypothyroidism for the past 15 years.  Her thyroid function studies have been in the normal range, and her levothyroxine dose has not been changed in 8 years.  She has no other medical issues and takes no other medications.
On examination, she appears to be slightly anxious.  Her blood pressure is 140/66 mm Hg and pulse is 98/min.  The thyroid gland is not palpable.  She has mild lid lag but no proptosis.  Fine hand tremors are present.  There is no pretibial myxedema.  The remainder of the physical examination is within normal limits.
Laboratory results are as follows:

Which of the following is the most likely cause of her thyrotoxicosis?

Quizplus · Accepted Answer

11ec193a_97b7_b7ef_853d_957422eda511_MD0002_00 Patients starting certain medications will need levothyroxine dose adjustments due to drug-drug interactions. Oral estrogens (e.g., birth control pills or hormonal replacement therapy) increase the sialylation of thyroxine-binding globulin (TBG), which causes decreased TBG clearance. Since TBG is the major T4 binding protein (produced in the liver), increased TBG concentrations lead to lower T4 levels and worsening hypothyroidism. As a result, patients started on oral estrogen preparations usually need an increased levothyroxine dose in order to saturate the increased number of TBG binding sites. This patient likely had her dose adjusted 13 years ago when she was started on estrogen. However, she stopped the estrogen on her own 4 months ago and continued the same dose of levothyroxine. The resulting decrease in circulating TBG combined with the unchanged levothyroxine dose likely resulted in iatrogenic thyrotoxicosis. (Choice A) Graves' disease is less likely in patients with longstanding hypothyroidism, since the thyroid gland is most likely completely burned out. (Choice B) Hashitoxicosis is rarely seen in patients with autoimmune thyroid disease. These patients usually present with transient thyrotoxicosis similar to Graves' followed by permanent hypothyroidism. (Choice D) Painless thyroiditis generally occurs in the postpartum period and would be unlikely in this non-postpartum patient with chronic autoimmune thyroiditis. (Choice E) Struma ovarii is the presence of thyroid tissue in an ovarian tumor (usually teratomas), which can hyperfunction and cause hyperthyroidism. This patient cannot have struma ovarii because her ovaries were removed. Educational objective: Oral estrogens increase the serum concentration of thyroid binding globulin (TBG). In normal patients, feedback loops promote increased endogenous production of thyroid hormones to compensate for the increased number of bound thyroid hormone molecules. However, patients with hypothyroidism cannot adequately compensate and an increased dose of levothyroxine is usually necessary. __________ References: Increased need for thyroxine in women with hypothyroid during estrogen replacement therapy (http://www.ncbi.nlm.nih.gov/pubmed/11396440)

A 63-Year-Old Woman Comes to the Physician Because of Hot