Changes in thyroid hormone levels during illness
| Severity of illness | TSH | Total T3 | Free T4 | Reverse T3 | Probable cause |
|---|---|---|---|---|---|
| Mild | No change | Mildly decreased | No change | Mildly increased | Mildly decreased D2, D1 |
| Moderate | No change or mildly decreased | Decreased | No change or mild increase or decrease | Increased | Decreased D2, D1, possibly mildly increased D3 |
| Severe | Decreased | Markedly decreased | Mildly decreased | Mildly increased | Decreased D2, D1, possibly mildly increased D3 |
| Recovery | Mildly increased | Mildly decreased | Mildly decreased | Mildly increased | Unknown |
D1 through D3 = iodothyronine deiodinases; T3, triiodothyronine; T4 = thyroxine; TSH = thyroid-stimulating hormone (thyrotropin).
Adapted from Salvatore D, Davies TF, Schlumberger M, Hay ID, Larsen PR. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 13th ed. Philadelphia, PA; Elsevier; 2016:334–368, copyright 2016, with permission from Elsevier.