This week, we are sharing another USMLE® Step 1-style practice question to test your knowledge of medical topics. Today’s case involves a 41-year-old woman with excessive fatigue and constipation. Can you figure it out?
A 41-year-old woman comes to the physician complaining of excessive fatigue for the last 2 weeks. She states that 1 month ago, she had experienced episodes of palpitations and diarrhea, but those symptoms have now resolved. Past medical history is significant for type I diabetes mellitus, which is treated with insulin. She also has been taking a laxative several times a week to help with new-onset constipation, which started 3 weeks ago. Temperature is 36.5°C (97.7°F), pulse is 48/min, blood pressure is 124/88 mm Hg, and BMI is 22 kg/m2. Physical examination shows cold, dry skin and thinning hair on the scalp. There is 1+ non-pitting edema on both lower extremities.
Which of the following additional findings is likely present in this patient?
A. Positive antimicrosomal antibodies
B. Human leukocyte antigen DQ8 positivity
C. Decreased thyroid stimulating hormone levels
D. Diffuse uptake of radioactive iodine on thyroid scintigraphy
E. Positive TSH-receptor antibodies
Scroll down to find the answer!
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The correct answer to today’s USMLE® Step 1 Question is…
A. Positive antimicrosomal antibodies
Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
The incorrect answers to today’s USMLE® Step 1 Question are…
B. Human leukocyte antigen DQ8 positivity
Incorrect: Human leukocyte antigen (HLA) DQ8 positivity is associated with celiac disease. Although this patient had episodes of diarrhea several weeks ago, the spontaneous resolution makes this diagnosis unlikely. This patient’s signs and symptoms make chronic autoimmune thyroiditis (Hashimoto thyroiditis) more likely, which is associated with HLA-DR5 positivity.
C. Decreased thyroid stimulating hormone levels
Incorrect: This patient is presenting with signs and symptoms of hypothyroidism (e.g., bradycardia, fatigue, hair thinning, non-pitting edema). The previous symptoms of palpitations and diarrhea were likely caused by autoimmune destruction of thyroid follicular cells and subsequent release of preformed T4 and T3. Therefore, the likely primary hypothyroidism seen in this patient would cause a resultant increase in thyroid stimulating hormone levels, not decrease.
D. Diffuse uptake of radioactive iodine on thyroid scintigraphy
Incorrect: Thyroid scintigraphy is an imaging technique that demonstrates the structure and function of thyroid tissue based on the selective uptake of radioactive iodine (RAI). Normally, only the functional part of the thyroid gland takes up RAI. In patients with a hyperfunctioning thyroid gland (e.g. Graves disease), there would be an enlarged thyroid gland with diffuse uptake of RAI. However, in this patient currently presenting with hypothyroidism, there would be decreased uptake.
E. Positive TSH-receptor antibodies
Incorrect: TSH-receptor antibodies are present in patients with Graves disease. The circulating TSH-receptor autoantibodies cause overstimulation of the thyroid gland, leading to excess production of thyroid hormone, resulting in signs of hyperthyroidism. In contrast, this patient is presenting with classical signs and symptoms of hypothyroidism, hence making Graves disease and the presence of TSH-receptor antibodies unlikely.
Main Explanation
This middle-aged female is presenting with constipation, dry skin, thinning hair, and non-pitting edema in the lower extremities. She most likely has hypothyroidism. Taking into account the history of a previously diagnosed autoimmune condition (e.g. type I diabetes mellitus) as well as prior symptoms of hyperthyroidism (e.g. episodes of palpitations and diarrhea), she most likely has chronic autoimmune thyroiditis (Hashimoto thyroiditis).
Hashimoto thyroiditis is the leading cause of hypothyroidism in the United States; it is autoimmune-mediated, and thus it tends to occur in patients with other underlying autoimmune conditions such as type I diabetes mellitus and vitiligo. Although the etiology is not fully understood, there is autoimmune-mediated lymphocytic inflammation and destruction of the thyroid tissue. This destruction of the follicular cells results in release of preformed thyroid hormones, which usually presents as an initial phase of hyperthyroidism. Patients subsequently progress to hypothyroidism as the thyroid parenchyma is destroyed and fibrosed.
Diagnosis is based on thyroid function tests, as well as the presence of antibodies, such as anti-thyroid peroxidase (anti-microsomal) and antithyroglobulin antibodies. Treatment is primarily with levothyroxine to replete the thyroid hormone deficiency.
Major Takeaway
Hashimoto thyroiditis is the result of an autoimmune-mediated lymphocytic inflammation and destruction of the thyroid tissue. Diagnosis is based on thyroid function tests, as well as the presence of certain antibodies, such as anti-thyroid peroxidase (anti-microsomal) and anti-thyroglobulin.
References
Caturegli, P., De Remigis, A., Rose, N.R. (2014) Hashimoto thyroiditis: clinical and diagnostic criteria. Autoimmunity Reviews. 13(4-5), 391–397. Doi: 10.1016/j.autrev.2014.01.007.
Chaker, L., Bianco, A.C., Jonklaas, J., Peeters, R.P. (2017) Hypothyroidism. Lancet. 390(10101), 1550-1562. Doi: 10.1016/S0140-6736(17)30703-1.
Fröhlich E., Wahl R. (2017) Thyroid autoimmunity: Role of anti-thyroid antibodies in thyroid and extra-thyroidal diseases. Frontiers in Immunology. 8, 521. Doi: 10.3389/fimmu.2017.00521.
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