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.