MKSAP Quiz: Evaluation of abnormal thyroid laboratory results
A 29-year-old woman is seen in follow-up for evaluation of abnormal thyroid laboratory results. She is currently 26 weeks pregnant. She was originally evaluated 1 week ago for concerns about lack of weight gain during pregnancy, palpitations, anxiety, and insomnia. After a physical exam and considering lab results, what is the most likely diagnosis?
A 29-year-old woman is seen in follow-up for evaluation of abnormal thyroid laboratory results. She is currently 26 weeks pregnant. She was originally evaluated 1 week ago for concerns about lack of weight gain during pregnancy, palpitations, anxiety, and insomnia. There is no family history of thyroid or autoimmune disease. Medical history is unremarkable, and her only medication is a prenatal vitamin.
On physical examination, other than a pulse rate of 98/min, vital signs are normal. The patient is a thin, gravid woman with a mild tremor of the outstretched hands, lid lag, and small goiter. There is no exophthalmus.
Laboratory studies obtained last week show a thyroid-stimulating hormone level of 6.5 µU/mL (6.5 mU/mL) and a free thyroxine (T4) level of 2.6 ng/dL (33.5 pmol/L).
Which of the following is the most likely diagnosis?
A. Gestational thyrotoxicosis
B. Graves disease
C. Hypothyroidism
D. Thyroid-stimulating hormone-secreting adenoma
MKSAP Answer and Critique
The correct answer is D. Thyroid-stimulating hormone-secreting adenoma. This content is available to MKSAP 18 subscribers as Question 34 in the Endocrinology and Metabolism section. More information about MKSAP is available online.
This patient most likely has a thyroid-stimulating hormone (TSH)-secreting adenoma. The initial evaluation based on clinical signs and/or symptoms of thyrotoxicosis should be measurement of serum TSH alone, followed by measurement of thyroxine (T4) and triiodothyronine (T3) levels if TSH is suppressed because the typical pattern of hyperthyroidism is TSH suppression with an elevated T4 and/or T3. However, this patient has signs and symptoms of hyperthyroidism with elevated TSH and free T4 levels, which is concerning for a TSH-secreting adenoma. These tumors are extremely rare and are managed differently from other causes of thyrotoxicosis. Before this diagnosis is made, other causes of the laboratory abnormalities must be excluded (thyroid hormone resistance and familial dysalbuminemic hyperthyroxinemia). If there is no other explanation for elevated TSH and T4 levels, a pituitary MRI should be performed. The patient's symptoms of hyperthyroidism and lack of family history of thyroid disease make a TSH-secreting adenoma more likely.
Serum TSH and human chorionic gonadotropin have a common α-subunit, allowing cross-reactivity at the TSH receptor. Gestational thyrotoxicosis typically occurs in the first trimester secondary to human chorionic gonadotropin (HCG) stimulation of the TSH receptor. Laboratory results would look similar to hyperthyroidism with suppressed TSH and elevated free T4.
The clinical manifestations of hypothyroidism include fatigue, cold intolerance, constipation, heavy menses, weight gain, impaired concentration, dry skin, edema, depression, mood changes, muscle cramps, myalgia, and reduced fertility. Hypothyroidism is unlikely as her symptoms are not compatible and her TSH levels would be elevated with a normal or low free T4 level.
The symptoms of thyrotoxicosis include heat intolerance, palpitations, dyspnea, tremulousness, menstrual irregularities, hyperdefecation, weight loss, increased appetite, proximal muscle weakness, fatigue, insomnia, and mood disturbances. The most common causes of hyperthyroidism are Graves disease and toxic adenoma(s). While the patient has thyrotoxicosis-related symptoms, hyperthyroidism due to Graves disease results in a suppressed TSH level with an elevated free T4 level, which is not found in this case.
Key Point
- Signs and symptoms of a thyroid-stimulating hormone-secreting adenoma are those seen in hyperthyroidism, although laboratory evaluation reveals an elevated free thyroxine (T4) level with an inappropriately normal or elevated thyroid-stimulating hormone level.