Hyperthyroidism Testing: Interpreting Thyroid Function Tests

Standard Thyroid Panel

Test	Hyperthyroidism Pattern	Notes
TSH	Suppressed (low/undetectable)	Most sensitive; pituitary reduces TSH in response to high T4/T3.
Free T4 (FT4)	Elevated	Active hormone; confirms hyperthyroidism if TSH low.
Free T3 (FT3)	Elevated	T3 toxicosis: FT3 high, FT4 normal, TSH low (early or iodine-induced).
Total T4 / Total T3	Elevated	Affected by TBG (pregnancy, OCPs, liver disease). Free levels preferred.

Additional Tests

• TSI (TSH Receptor Antibodies): Positive in Graves' disease (90–95%).
• TPOAb, TgAb: Positive in Hashimoto's (can have hashitoxicosis phase).
• Radioactive Iodine Uptake (RAIU) + Scan: Differentiates Graves' (diffuse ↑ uptake), toxic multinodular goiter (patchy), thyroiditis (low uptake), factitious (low uptake).
• Inflammatory Markers: ESR/CRP elevated in subacute thyroiditis.

Interpretation Algorithm

1. Low TSH → check FT4, FT3.
2. High FT4/FT3 → hyperthyroidism confirmed.
3. TSI + RAIU → etiology.
4. If TSH low but FT4/FT3 normal → subclinical hyperthyroidism; monitor or treat if high risk.

Always interpret in clinical context (symptoms, medications, pregnancy).