A 26-year-old woman comes to the office for a health maintenance visit. The patient feels well except for increasing fatigue over the past 3 months. She has a history of hypothyroidism and has been treated with a stable dose of levothyroxine for the past several years. The patient has no other medical conditions and has had no surgeries. She does not use tobacco, alcohol, or illicit drugs, and consumes a balanced diet. The patient is sexually active with her boyfriend and recently began taking a combination oral contraceptive for birth control. Vital signs are normal. The thyroid is normal to palpation. Cardiopulmonary and abdominal examinations show no abnormality. Thyroid function studies reveal significant changes compared to testing performed 6 months ago, and the changes are attributed to the oral contraceptive. This medication most likely altered which of the following aspects of thyroid function in this patient?

Question

A 26-year-old woman comes to the office for a health maintenance visit.  The patient feels well except for increasing fatigue over the past 3 months.  She has a history of hypothyroidism and has been treated with a stable dose of levothyroxine for the past several years.  The patient has no other medical conditions and has had no surgeries.  She does not use tobacco, alcohol, or illicit drugs, and consumes a balanced diet.  The patient is sexually active with her boyfriend and recently began taking a combination oral contraceptive for birth control.  Vital signs are normal.  The thyroid is normal to palpation.  Cardiopulmonary and abdominal examinations show no abnormality.  Thyroid function studies reveal significant changes compared to testing performed 6 months ago, and the changes are attributed to the oral contraceptive.  This medication most likely altered which of the following aspects of thyroid function in this patient?

Quizplus · Accepted Answer

Explanation: 11ec2b69_c13d_dd11_8f63_01e8fb365822_MD0005_00 More than 99% of circulating thyroid hormone (T4, T3) is bound to plasma proteins. The largest share is carried by thyroxine-binding globulin (TBG), with additional carriers including transthyretin, albumin, and other proteins. Biologically active, free thyroid hormone is rapidly cleared by the kidneys, and so binding proteins ensure that adequate hormone is available for delivery to the peripheral tissues. Elevated estrogen levels, as seen in pregnancy and patients taking oral contraceptives, stimulate hepatic synthesis of TBG. Patients with normal thyroid function can increase thyroid hormone production to saturate the additional protein binding sites, but those with primary hypothyroidism are unable to increase thyroid hormone synthesis; oral contraceptive use in these patients can induce a relative hypothyroid state. Most patients will therefore require an increased dose of levothyroxine when starting oral contraceptives or becoming pregnant. Factors that conversely decrease TBG include androgenic steroids, glucocorticoids, chronic liver disease, and protein-losing states (eg, nephrotic syndrome). Salicylates do not have a significant effect on plasma protein levels but alter thyroid physiology by displacing thyroid hormone from protein binding sites. (Choice A) Bile acid-binding resins (eg, cholestyramine) and some dietary supplements (iron, fiber, antacids) decrease intestinal levothyroxine absorption; levothyroxine should be taken 4 hours before or after these substances. (Choice B) Multiple medications can interfere with thyroid hormone levels by either inducing (eg, barbiturates) or inhibiting (eg, amiodarone) hepatic metabolism of thyroid hormones. Patients taking these medications may require modification of levothyroxine dosage to maintain a euthyroid state. (Choice C) Glucocorticoids, beta blockers, and propylthiouracil inhibit peripheral deiodination of T4 to T3. These agents are sometimes used in the treatment of thyrotoxicosis and thyroid storm. (Choice D) Lithium, potassium iodide, and Lugol solution (elemental iodine and potassium iodide) inhibit thyroid hormone secretion. Estrogen, however, does not affect thyroid hormone secretion. Educational objective: Most thyroid hormone in the circulation is bound to plasma proteins, primarily thyroxine-binding globulin (TBG). Estrogen stimulates hepatic synthesis of TBG. Hypothyroid patients with elevated estrogen levels (eg, pregnancy, oral contraceptive use) require a higher dose of levothyroxine to saturate the increased number of binding sites. References: Effects of a monophasic combined oral contraceptive containing nomegestrol acetate and 17ß-oestradiol in comparison to one containing levonorgestrel and ethinyl estradiol on markers of endocrine function. (http://www.ncbi.nlm.nih.gov/pubmed/21942708) Effects of the environment, chemicals and drugs on thyroid function. (http://www.ncbi.nlm.nih.gov/pubmed/25905415)

A 26-Year-Old Woman Comes to the Office for a Health