Specialized Diagnostic Tests in Thyroid Disorders

Total T4

Total serum concentrations (∴ influenced by TBG). Checked if concern that TSH and free T4 are not accurate.

Free T3

Unbound T3, low clinical utility

Reverse T3

Inactive, ↑’d in sick euthyroid syndrome. Rarely used clinically.

Thyroid stimulating Abs

Thyroid-stimulating Ig (TSI) and thyrotropin-binding inhibitory immunoglobulin (TBII) seen in Graves’ disease. Diagnostic of Graves’ disease in high titer.

Thyroglobulin

↑’d in goiter, hyperthyroidism and thyroiditis ↓’d in factitious ingestion of thyroid hormone Tumor marker for thyroid cancer only after total thyroidectomy and radioiodine therapy

Thyroxine-binding globulin (TBG)

↑ TBG (∴ ↑ T4): estrogen (OCP, preg.), hepatitis, opioids, hereditary ↓ TBG (∴ ↓ T4): androgens, glucocorticoids, nephrotic syndrome, cirrhosis, acromegaly, antiepileptics, hereditary

Radioactive iodine uptake (RAIU) scan

• Useful to differentiate causes of hyperthyroidism
  • ↑ uptake: Graves’ disease, toxic multinodular goiter or hot nodule
  • Gold: no uptake: subacute thyroiditis painful (de Quervain’s) or silent thyroiditis, exogenous thyroid hormone, recent iodine load, struma ovarii or antithyroid drugs