Sick-euthyroid syndrome

**Long definition:** The term sick-euthyroid syndrome may occur in fetal life, acute or chronic illness, exhausting exercise or starvation, which don't result from a thyroid disorder, and which may be present separately or in combination:

1) Primary low-T3 syndrome resulting from hypodeiodination

2) Secondary low-T3 syndrome caused by low-T4 syndrome

3) Thyrotropic adaptation resulting in low-TSH-low-T4 syndrome

4) Impaired plasma protein binding of thyroid hormones

5) Increased concentrations of reverse T3

6) Increased concentrations of 3,5-diiodothyronine (3,5-T2)

7) Measurement artifacts resulting from pathological blood components including free fatty acids, antibodies and drugs.