Thyroid Function Tests

TSH  (Thyroid Stimulating Hormone)

  • Main Screening Test
  • High =  Hypothyroidism
  • Low  =  Hyperthyroidism
  • Normal values drop in Pregnancy (see “Hypothyroidism”)
FT4  (Free T4)  [a.k.a. “Free Thyroxine”]
  • Main follow-up test for abnl TSH
  • High =  Hyperthyroidism
  • Low  =  Hypothyroidism
T4  (i.e. Total T4)
  • Less reliable in general
  • High =  Hyperthyroidism
  • Low  =  Hypothyroidism
T3  [a.k.a. Triiodothyronine]
  • Only ordered in context of Hyperthyroidism
Free Thyroxine Index  [“FTI”]
  • Calculation derived from Total T4 + T3 Resin Uptake
  • Useless today, because FT4 readily available
  • Better than Total T4 if FT4 too expensive
T3 Resin Uptake
  • Nothing to do with T3
  • An ingredient in FTI calculation [see above]