A 24-year-old primigravida comes to the office for her first prenatal visit. Her last menstrual period was 8 weeks ago and 2 home pregnancy tests have been positive. The patient feels generally well, although she has experienced mild intermittent nausea and anxiety during the pregnancy. Medical history is unremarkable, and her only medication is an over-the-counter prenatal vitamin. Thyroid function tests were normal on evaluation for fatigue and mild palpitations 6 months ago. The patient stopped drinking alcohol after finding out she was pregnant, and she does not use tobacco or illicit drugs. Blood pressure is 120/80 mm Hg and pulse is 82/min. Physical examination findings are normal. Pelvic ultrasound shows a gestational sac containing a fetal pole and a yolk sac. Thyroid function study results are as follows: Which of the following is the most likely cause of this patient's current laboratory findings?

Question

A 24-year-old primigravida comes to the office for her first prenatal visit.  Her last menstrual period was 8 weeks ago and 2 home pregnancy tests have been positive.  The patient feels generally well, although she has experienced mild intermittent nausea and anxiety during the pregnancy.  Medical history is unremarkable, and her only medication is an over-the-counter prenatal vitamin.  Thyroid function tests were normal on evaluation for fatigue and mild palpitations 6 months ago.  The patient stopped drinking alcohol after finding out she was pregnant, and she does not use tobacco or illicit drugs.  Blood pressure is 120/80 mm Hg and pulse is 82/min.  Physical examination findings are normal.  Pelvic ultrasound shows a gestational sac containing a fetal pole and a yolk sac.  Thyroid function study results are as follows:  Which of the following is the most likely cause of this patient's current laboratory findings?

Quizplus · Accepted Answer

Explanation: 11ec25c8_d999_4526_bc81_efb1c0820403_MD0003_00 Thyroid hormone production increases during pregnancy to cope with metabolic demands. This is accomplished by 2 mechanisms: Estrogen stimulates synthesis of thyroxine-binding globulin (TBG) and decreases TBG clearance, leading to an increased pool of bound thyroid hormone. Patients with normal thyroid reserve subsequently increase thyroid hormone production to maintain free hormone levels. hCG (which shares a common alpha subunit with TSH and a very similar beta subunit) directly stimulates TSH receptors. This results in increased hormone production with feedback suppression of pituitary TSH release. In patients with normal thyroid function, the net effect (as seen in this patient) can resemble subclinical hyperthyroidism with increased circulating total T3 and T4, normal (or mildly elevated) free T4, and suppressed TSH, although the patient remains clinically euthyroid. However, patients with impaired thyroid function (eg, chronic lymphocytic [Hashimoto] thyroiditis) cannot increase thyroid hormone production, resulting in a relative hypothyroid state (Choice C). For this reason, patients with baseline hypothyroidism who become pregnant should have their replacement thyroxine dose increased. (Choice A) Euthyroid sick syndrome is an alteration in biochemical thyroid function tests in the setting of severe nonthyroid illness. Common findings include a low total and free T3 with a normal T4 and TSH. (Choice B) Graves disease causes an increase in total and free thyroid hormone levels. However, most patients will have additional manifestations, such as proptosis, diffuse goiter, hypertension, and tachycardia. (Choice D) Iodine requirements increase during pregnancy, and inadequate iodine intake can cause maternal and fetal hypothyroidism. Such patients would have an elevated TSH with decreased thyroid hormone levels. (Choice F) Subacute (granulomatous or De Quervain) thyroiditis can cause hyperthyroidism with a suppressed TSH. However, it most commonly follows an acute viral illness and is associated with a painful, tender goiter. Educational objective: Thyroid hormone production increases during pregnancy to cope with metabolic demands. Estrogen causes an increase in thyroxine-binding globulin, leading to increased total (but not free) thyroid hormone levels. hCG directly stimulates TSH receptors, causing increased production of thyroid hormones. References: Thyroid disease in pregnancy: (Women's Health Series). (http://www.ncbi.nlm.nih.gov/pubmed/24002560) Practice bulletin no. 148: thyroid disease in pregnancy. (http://www.ncbi.nlm.nih.gov/pubmed/25798985) Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. (http://www.ncbi.nlm.nih.gov/pubmed/21787128)

A 24-Year-Old Primigravida Comes to the Office for Her First