99% of thyroxine is predominantly bound to thyroxin binding globulin (TBG). Free T4 does not bind to transport proteins but is free in the circulation and is unbound or metabolically available. This unbound fraction, or free T4 (FT4) is a precursor to Triiodothyronine (T3).
Please refer to clinical details on TSH to review overall causes of thyroid disorders.
Tests Results
| Raised TSH, low T4 | Hypothyroidism |
| Raised TSH, normal T4 | Consistent with subclinical hypothyroidism |
| Raised TSH, raised T4 | Thyroid hormone resistance, rarely TSH tumour. |
| Low TSH, raised T3 or T4 | Hyperthyroidism |
| Low TSH, normal T3, T4 | Subclinical hyperthyroidism |
| Low TSH, low T3 and T4 | Sick euthyroid disease, or pituitary disease. |
