Total Thyroxine (T4)
Thyroid marker
Total T4
Total Thyroxine (T4)
The total amount of thyroxine (T4) in serum, including both the small free (active) fraction and the large fraction bound to carrier proteins such as thyroxine-binding globulin (TBG), transthyretin, and albumin. Because it measures bound plus free hormone, Total T4 moves whenever binding-protein levels change, unlike Free T4 which reflects only the unbound hormone.
PED Notes
Total T4 is heavily influenced by binding proteins, so it can shift without any true change in thyroid status. Oral oestrogens (and the altered hepatic protein synthesis seen with some AAS use) raise TBG and push Total T4 up, while high-dose androgens can lower TBG and Total T4, in both cases with normal Free T4 and normal thyroid function. Pregnancy raises it too. For monitoring thyroid status on cycle, TSH plus Free T4 (or Free T3) are more reliable than Total T4. Total T4 is mainly useful as part of the legacy Total T4 x T3 Uptake calculation (the Free T4 Index) when a direct Free T4 is unavailable.
When high
When Total T4 is high:
- First distinguish true hyperthyroidism from a binding-protein artefact: check TSH and Free T4. A suppressed TSH with high Free T4 points to genuine hyperthyroidism (Graves' disease, thyroiditis, excess exogenous thyroid hormone).
- A high Total T4 with normal TSH and normal Free T4 usually reflects raised TBG (oral oestrogen, pregnancy, some AAS-related hepatic changes) and needs no thyroid treatment.
- If genuinely hyperthyroid, refer to an endocrinologist and review any T3/T4 supplementation used for fat loss.
When low
When Total T4 is low:
- Confirm with TSH and Free T4 before acting. A low Total T4 with normal TSH and normal Free T4 commonly reflects low TBG (high-dose androgens, nephrotic protein loss, illness) rather than hypothyroidism.
- A low Free T4 with a high TSH indicates true primary hypothyroidism: see the Free T4 and TSH markers for the full supplement and levothyroxine approach.
- Prolonged aggressive contest-prep dieting lowers thyroid output; restoring energy intake often normalises values.
History Chart
Reading History
Frequently Asked Questions
Reference Ranges
Standard Range
VitalMetrics Range