In a patient with insomnia and tachycardia, which diagnosis is most likely and which lab finding would confirm it?

In a patient with insomnia and tachycardia, which diagnosis is most likely and which lab finding would confirm it?

• A. Hypothyroidism; Low TSH
• B. Hyperthyroidism; High TSH
• C. Hyperthyroidism; Low TSH
• D. Hyperthyroidism; Normal TSH

Answer: (C)

The key idea is that signs like insomnia and tachycardia reflect an increased metabolic and sympathetic state from excess thyroid hormone. In primary hyperthyroidism, the elevated thyroid hormones (T3/T4) sweep back to the pituitary and shut down TSH production, so TSH becomes low. That’s why a low TSH level would confirm the diagnosis in this clinical context, typically alongside elevated free T4/T3. Hypothyroidism would usually cause bradycardia and a high TSH, not tachycardia and insomnia, and a normal or high TSH would not fit the hyperthyroid picture. So the best match is hyperthyroidism with a low TSH.

The key idea is that signs like insomnia and tachycardia reflect an increased metabolic and sympathetic state from excess thyroid hormone. In primary hyperthyroidism, the elevated thyroid hormones (T3/T4) sweep back to the pituitary and shut down TSH production, so TSH becomes low. That’s why a low TSH level would confirm the diagnosis in this clinical context, typically alongside elevated free T4/T3. Hypothyroidism would usually cause bradycardia and a high TSH, not tachycardia and insomnia, and a normal or high TSH would not fit the hyperthyroid picture. So the best match is hyperthyroidism with a low TSH.