Chemistry
Thyroid related
Thyroxine-free (free T4)

Author: Nat Pernick, M.D. (see Authors page)

Revised: 23 February 2016, last major update March 2011

Copyright: (c) 2002-2016, PathologyOutlines.com, Inc.

PubMed Search: Thyroxine-free [title]
Pathophysiology
  • Biologically active fraction of T4 in circulating blood (0.03% of total T4)
  • Patients appear to have a genetically determined free T4 setpoint with significant individual variation
  • Initially, TSH secretion shows marked changes in response to small free T4 changes
  • Previously estimated by free thyroxine index
Laboratory
Indications
  • Evaluate thyroid function
  • Free T4 together with TSH is most accurate assessment of thyroid status
  • Free T4 is better than total T4 since:
    • Free T4 reflects biologically active hormone
    • Free T4 is not affected by changes in thyroid binding proteins or their binding characteristics

Methodology
  • High-affinity hormone antibody, immunoassay or equilibrium dialysis
  • High affinity hormone antibody:
    • Used on most automated platforms
    • Measures fractional occupancy of hormone antibody-binding sites
    • Free hormone concentration is inversely proportional to number of unoccupied antibody binding sites
    • Quantify using hormone labeled with fluorescence or chemiluminescence
    • Convert to free hormone levels using calibrators
  • Immunoassay:
    • Commonly used
    • Affected by changes in binding protein concentration or nonthyroidal illness (Clin Chem 2007;53:985)
  • Equilibrium dialysis:
    • Not affected by changes in binding protein concentration or nonthyroidal illness
    • Time consuming / not practical for most laboratories
    • Dialyze sample in nonprotein buffer against ultrafiltratable membrane, until free T4 reaches equilibrium on both sides of membrane, then analyze T4 in protein-free solution

Reference ranges
  • Serum: 0.7 - 1.7 ng/dl (up to 2.2 ng/dl at 0 - 30 days of life)
  • Amniotic fluid (Bayer ADVIA Centaur): less than 0.10 to 0.77 ng/dL (1.29 - 9.93 pmol/L, (Am J Clin Pathol 2007;128:158)

Limitations
  • Results of high affinity hormone antibody or immunoassay may be spurious if binding globulin abnormality, T3 or T4 autoantibodies (J R Soc Med 2003;96:50), pregnancy, nonthyroidal illness, carbamazepine or phenytoin, premature infants (J Perinatol 2004;24:640)
  • Recommended to use equilibrium dialysis methodology
  • Within laboratory variation is frequently high (Arch Pathol Lab Med 2005;129:318)
Interpretation
  • Increased:
    • Graves disease, toxic multinodular goiter or toxic adenoma, iatrogenic / factitious
    • Transient increase in subacute thyroiditis or Hashimoto thyroiditis
    • Rarely increased in thyroid cancer, secondary to amiodarone, secondary to pituitary disease
  • Decreased:
    • Primary, secondary or tertiary hypothyroidism, tissue resistance to thyroid hormone