What this test measures
Thyroxine (T4) is the principal hormone secreted by the thyroid gland — the storage and supply form. The "total" T4 assay measures both the protein-bound (about 99.97%) and the free (about 0.03%) fractions together. Because most circulating T4 is protein-bound, total T4 levels vary with carrier protein levels (thyroxine-binding globulin, transthyretin, albumin).
This is a strength in simple outpatient settings and a weakness in pregnancy, on oestrogen pills, in liver disease and in nephrotic syndrome — situations where Free T4 is preferred.
Why it matters
Total T4 is almost always ordered together with TSH (and often T3). A coherent pattern — high TSH with low T4, or low TSH with high T4 — is straightforward primary thyroid disease. Discordant patterns (low T4 with non-raised TSH; high T4 with non-suppressed TSH) flag unusual conditions.
For most uncomplicated outpatient screens, the Total TFT bundle is sufficient and cheaper. Free T4 is preferred where binding proteins are likely abnormal. Total T4 as a standalone test (without TSH) is uncommon and limited to specific endocrine workups.
How to prepare
No fasting required. Morning sample preferred for consistency. If on levothyroxine, take it after the blood draw. Stop biotin / cosmetic supplements 48–72 hours before. Continue other medications.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Total Thyroxine (T4) (µg/dL)[1][2] | 5.0 – 12.0 µg/dL (64 – 154 nmol/L) | Low T4 with raised TSH = primary hypothyroidism. Low T4 with low / inappropriately normal TSH = central hypothyroidism (pituitary or hypothalamic) — needs imaging. Low T4 with normal TSH can also be T4-binding protein deficiency or sick euthyroid syndrome. | Raised T4 with suppressed TSH = primary hyperthyroidism (Graves', toxic nodule, thyroiditis). Raised total T4 with normal TSH and normal free T4 in pregnancy or on oestrogen pills = increased binding protein, not true hyperthyroidism. Raised T4 with non-suppressed TSH = rare TSH-secreting pituitary adenoma or thyroid hormone resistance. |
Total T4 + TSH — pattern recognition
| TSH | Total T4 | Likely picture |
|---|---|---|
| Raised | Low | Overt primary hypothyroidism |
| Raised | Normal | Subclinical hypothyroidism |
| Suppressed | Raised | Overt primary hyperthyroidism |
| Suppressed | Normal | Subclinical hyperthyroidism or T3 toxicosis |
| Low or normal | Low | Central hypothyroidism |
| Normal | Raised | Pregnancy / OCP — check Free T4 |
| Raised | Raised (rare) | TSH-oma or hormone resistance |
Frequently asked questions
When is Total T4 preferred over Free T4?
In uncomplicated outpatient screens where cost matters and binding proteins are likely normal. Free T4 is preferred in pregnancy, on oestrogen pills, in severe liver disease and in nephrotic syndrome.
My T4 is high but I feel fine — is it serious?
Not necessarily. Pregnancy, oestrogen pills and HRT raise total T4 by raising binding proteins, without true hyperthyroidism. Free T4 will be normal in this case. Check the TSH to clarify.
Do I need to fast?
No. A morning sample is preferred for consistency.
Should I take levothyroxine on the test day?
Take it after the blood is drawn. Taking it 1–2 hours before can transiently raise T4 readings.
Why is Total T4 less useful in pregnancy?
Pregnancy oestrogen raises thyroid-binding globulin, lifting total T4 by 30–50% even when thyroid function is normal. Free T4 gives the truer picture in pregnancy.
Will biotin affect Total T4?
Yes — high-dose biotin in hair-skin-nail supplements typically gives falsely high Total T4 and Free T4 with falsely low TSH. Stop 48–72 hours before.
My T4 is low and TSH is normal — what does that mean?
Low T4 with non-raised TSH suggests central hypothyroidism (pituitary or hypothalamic problem) — needs pituitary imaging and a full pituitary hormone panel.
Related Hormones / Endocrine tests
Tests commonly ordered alongside TOTAL THYROXINE (T4), or that help interpret an unexpected result.
Sources & references
- American Thyroid Association — Thyroid Function Tests · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — T4 Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Thyroxine · accessed 2026-05-30T00:00:00.000Z
- Indian Thyroid Society — Consensus · accessed 2026-05-30T00:00:00.000Z
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