Which thyroid function pattern indicates hyperthyroidism?

• A. TSH < 0.01 mIU/L with FT4 elevated
• B. TSH > 4.5 mIU/L with FT4 elevated
• C. TSH normal with FT4 elevated
• D. TSH suppressed with FT4 normal

Answer: (A)

When thyroid hormone production is high, it feeds back to the pituitary to shut down TSH. The most telling sign of overt hyperthyroidism is a very suppressed TSH together with an elevated free T4. That combination—TSH falling below the detection limit and FT4 being high—shows the thyroid is overactive and the pituitary is actively reducing TSH in response to the excess hormone.

If FT4 is elevated but TSH remains normal or high, that would be unusual for primary thyroid overactivity and could suggest a pituitary issue that’s driving TSH, rather than typical hyperthyroidism. If TSH is suppressed but FT4 is normal, that points to subclinical hyperthyroidism, not overt hyperthyroidism. So the pattern of low TSH with elevated FT4 is the classic biochemical signature of hyperthyroidism.