Medications that distort in vitro tests of thyroid function, with particular reference to estimates of serum free thyroxine.
The combination of serum thyroid-stimulating hormone (TSH) with measurement of circulating thyroid hormones greatly improves sensitivity and specificity of thyroid diagnosis, but these assays are not impeccable. Estimation of serum free T4 conveniently accommodates variations in the concentration of thyroxine-binding globulin (TBG), but no current technique reliably reflects the in vivo free T4 concentration in numerous other situations. The effect of circulating competitors that increase T4 and T3 in vivo, in particular, many medications, is under-estimated by current free hormone estimates that involve sample dilution. Non-esterified fatty acids generated during sample storage and incubation can spuriously increase the measured free T4 estimate, especially after in vivo treatment with heparin. These artefacts are unlikely to be overcome by current assay strategies. Total serum T4, corrected for alterations in TBG concentration, gives a more robust estimate of thyroxine concentration than current methods of free hormone estimation and should now be reintroduced as the 'gold standard'.
Stockigt
Jim R
JR
Monash University, Melbourne, Australia. jrs@netspace.net.au
Lim
Chen-Fee
CF
D000910:Antibodies, Heterophile / Q000097:blood; D016477:Artifacts; D003951:Diagnostic Errors; D004355:Drug Stability; D005230:Fatty Acids, Nonesterified / Q000097:blood; D005665:Furosemide / Q000032:analysis; D006801:Humans; D013048:Specimen Handling / Q000009:adverse effects; D013960:Thyroid Function Tests / Q000379:methods*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood*; D056825:Thyroxine-Binding Globulin; D014284:Triiodothyronine / Q000097:blood
D000910:Antibodies, Heterophile; D005230:Fatty Acids, Nonesterified; D056825:Thyroxine-Binding Globulin; D014284:Triiodothyronine; D005665:Furosemide; D013972:Thyrotropin; D013974:Thyroxine