Suspected hyperthyroidism

Free T₃ (elevated)

Thyroid scan

Decreased RAIU

End

Rule out thyrotoxicosis factitia (e.g., ingestion of levothyroxine)

RAIU scan, serum thyroglobulin level

Hyperthyroidism

Decreased RAIU, increased serum thyroglobulin

Subacute thyroiditis

RAIU scan

Decreased uptake

Autoimmune lymphocytic thyroiditis

Graves disease or toxic nodule

RAIU scan

Low

Elevated

Rule out iatrogenic ingestion of liothyronine

Rule out T₃ thyrotoxicosis

Thyroid gland tender

Thyroid gland not enlarged, no nodules

Thyroid gland enlarged

RAIU, Radioactive iodine uptake; TSH, thyroid-stimulating hormone.

TSH, free T₄

Low free T₄, normal/low TSH

Elevated free T₄, low TSH