Thyroid function test results in patients with nonthyroidal illness
| Thyroid test | Result | Mechanisms |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | Normal, high, or low | Low TSH because of: • Suppression of hypothalamic-pituitary axis by inflammatory cytokines • Abnormal TSH glycosylation • Decreased leptin resulting in low thyrotropin-releasing hormone, resulting in low TSH • Increased hypothalamic and pituitary type 2 iodothyronine deiodinase (D2) activity resulting in increased local T3 and thus decreased TSH Transient TSH increase during recovery from acute illness can be seen |
| Serum free thyroxine (T4) | Normal, high, or low | Increased “direct” free T4 possibly because of inhibitors of T4 to its binding proteins6 Decreased free T4 index possibly because of very low binding protein concentrations7 |
| Total T4 | Normal or low | Decreased total T4 because of: • Low production of its binding proteins • Decreased binding to thyroxine-binding globulin (inhibitors of T4 binding, glycosylated thyroxine-binding globulin) • Low TSH |
| Total triiodo-thyronine (T3) | Low | Decreased type 1 iodothyronine deiodinase (D1) activities (by cytokines, increased cortisol, free fatty acids, and drugs) Low production of thyroxine-binding globulin Decreased binding to thyroxine-binding globulin (inhibitors of T3 binding, glycosylated thyroxine-binding globulin) Low TSH |
| Reverse T3 | Higha | Increased type 3 iodothyronine deiodinase (D3) activity Decreased D1 activity |
↵a Except in patients with end-stage renal disease and in some patients with acquired immune defi ciency syndrome.