MKSAP Quiz: 2-week history of heat intolerance in pregnancy
A 32-year-old woman is evaluated in the first trimester of pregnancy for a 2-week history of heat intolerance, palpitations, and tremulousness. Her only medication is folic acid. Following a physical examination and laboratory studies, what is the most appropriate diagnostic test?
A 32-year-old woman is evaluated in the first trimester of pregnancy for a 2-week history of heat intolerance, palpitations, and tremulousness. Her only medication is folic acid.
On physical examination, pulse rate is 110/min; remaining vital signs are normal. The thyroid is nontender and symmetrically and diffusely enlarged. A fine tremor on the patient's outstretched hands is noted.
Laboratory studies show a thyroid-stimulating hormone level of less than 0.01 μU/mL (0.01 mU/L) and free thyroxine level of 5.3 ng/dL (68 pmol/L).
Which of the following is the most appropriate diagnostic test?
A. Thyroid scintigraphy with radioactive iodine uptake
B. Thyroid-stimulating immunoglobulin measurement
C. Thyroid ultrasonography
D. Total triiodothyronine measurement
MKSAP Answer and Critique
The correct answer is B. Thyroid-stimulating immunoglobulin measurement. This content is available to MKSAP 19 subscribers as Question 14 in the Endocrinology and Metabolism section. More information about MKSAP is available online.
The most appropriate diagnostic test is thyroid-stimulating immunoglobulin (TSI) measurement (Option B). The diagnosis of hyperthyroidism is based on biochemical testing demonstrating a low serum thyroid-stimulating hormone (TSH) level and elevated concentrations of free thyroxine (T4) and/or total triiodothyronine (T3). Thyroid scintigraphy with radioactive iodine uptake (RAIU) can verify the cause. Additional testing can be done when the clinical diagnosis is unclear; when RAIU is unavailable or unreliable, such as in patients taking amiodarone or lithium or those recently exposed to iodinated contrast material; or when scintigraphy is contraindicated, such as in pregnancy and lactation. In the absence of RAIU, additional tests include measurement of TSI or thyrotropin receptor antibodies (TRAb). In this patient, TSI measurement is a reasonable first test because an abnormal result has prognostic and treatment implications. Other possible causes of the patient's hyperthyroidism include human chorionic gonadotropin (hCG)-mediated hyperthyroidism and thyroiditis. Because hCG stimulates thyroid hormone secretion, TSH may be mildly suppressed as a result. Serum TSH gradually returns to the nonpregnant reference range in the second and third trimester. Determining the cause of hyperthyroidism is important because transient causes of pregnancy-related hyperthyroidism, such as hCG-mediated hyperthyroidism and thyroiditis, may not require intervention other than laboratory monitoring.
Thyroid scintigraphy with RAIU (Option A) is contraindicated in pregnancy. In this patient, measurement of TSI or TRAb is preferred to evaluate the possibility of Graves disease.
Thyroid ultrasonography (Option C) may help identify thyroid nodules not detected on physical examination but would not provide information about whether they are the cause of thyroid dysfunction. Doppler studies with thyroid ultrasonography may show increased vascularity indicative of thyroid hyperfunction or decreased vascularity indicative of thyroiditis or exogenous thyroid use. This imaging technique alone is insufficiently specific to guide management.
Total T3 measurement (Option D) may be useful in identifying clinical thyrotoxicosis in the setting of a normal free T4 level, although total T3 levels are increased in pregnancy because of changes in thyroid-binding globulin. In this patient, however, the free T4 level is undisputedly elevated and the total T3 would not add any useful information.
Key Points
- Possible causes of thyrotoxicosis in pregnant persons include human chorionic gonadotropin-mediated hyperthyroidism, Graves disease, and thyroiditis.
- In the diagnosis of Graves disease, measurement of thyroid-stimulating immunoglobulin or thyrotropin receptor antibodies can be helpful if thyroid scintigraphy with radioactive iodine uptake is unavailable, unreliable, or contraindicated.