Include name and telephone number of contact physician.
1. Draw blood immediately before next scheduled dose.
2. Centrifuge and aliquot serum into plastic vial within 2 hours of draw.
Certain drugs may cause short-term free thyroxine fluctuations.
- Acetylsalicylic acid (aspirin)
- Salicylic acid (salsalate)
All free hormone assays should be combined with thyrotropin (thyroid-stimulating hormone) measurements.
Free thyroxine (FT4) levels below 0.8 ng/dL indicate possible hypothyroidism. FT4 levels above 2.0 ng/dL indicates possible hyperthyroidism.
Neonates can have significantly higher FT4 levels. The hypothalamic-pituitary-thyroid axis can take several days or, sometimes, weeks to mature.