Summary
Free T4 is the primary hormone secreted by the thyroid gland and the key indicator of thyroid output. Unlike total T4, Free T4 measures only the biologically active, unbound fraction — giving a true picture of how much thyroid hormone is actually available to your cells and tissues.
Free T4 is most informative interpreted alongside TSH. Raised TSH with low Free T4 confirms overt hypothyroidism. Suppressed TSH with elevated Free T4 confirms hyperthyroidism. Normal Free T4 with an abnormal TSH may indicate subclinical thyroid disease where early intervention can prevent progression.
For people on levothyroxine, Free T4 monitoring helps confirm replacement doses are achieving adequate circulating levels. Some individuals need Free T4 in the upper half of the reference range to feel well.
What It Is
T4 is synthesised in the thyroid gland using iodine and tyrosine. About 99% circulates bound to carrier proteins (thyroxine-binding globulin, albumin, transthyretin). Only the unbound fraction — Free T4 — is biologically active and can enter cells.
Once inside cells, T4 is converted to the more potent hormone Free T3 by enzymes called deiodinases. T4 therefore acts primarily as a circulating reservoir and prohormone for T3. This conversion occurs mainly in the liver, kidneys, and muscle.
Free T4 is more reliable than total T4 because it is not affected by changes in binding proteins caused by pregnancy, oral contraceptives, or liver disease.
Functions
Metabolic rate regulation
Drives basal metabolic rate by stimulating oxygen consumption and heat production across virtually all body cells.
Cardiovascular function
Regulates heart rate, cardiac output, and vascular tone. Low Free T4 is associated with bradycardia and elevated cholesterol.
Neurological development and cognition
Essential for foetal brain development; supports cognitive speed, mood, and neurological function in adults.
Prohormone for Free T3
Serves as the primary substrate converted to biologically active Free T3 by deiodinase enzymes in peripheral tissues.
Reference Ranges
Serum Free T4
Measured in pmol/L| Status | Range (pmol/L) | What it means |
|---|---|---|
| Low | < 12 | Indicates reduced thyroid output — consistent with hypothyroidism when TSH is also elevated. |
| Borderline | 12–15 | Below mid-range; may not provide symptom relief for some individuals on treatment. |
| Normal | 15–22 | Adequate thyroid output — thyroid and pituitary functioning in balance. |
| Elevated | > 22 | Excess thyroid hormone — investigate for hyperthyroidism or over-treatment. |
Free T4 ranges differ between laboratory assays and should always be interpreted alongside TSH and clinical symptoms.
Symptoms of Imbalance
Free T4 symptoms mirror hypo- and hyperthyroidism depending on whether levels are low or elevated.
- Extreme fatigue and persistent low energy
- Unexplained weight gain despite normal diet
- Cold intolerance and low body temperature
- Constipation
- Dry skin, hair loss, and brittle nails
- Slow heart rate
- Brain fog and difficulty concentrating
- Palpitations and rapid or irregular heartbeat
- Unintentional weight loss
- Anxiety, nervousness, and irritability
- Heat intolerance and excessive sweating
- Loose stools or diarrhoea
- Tremor and muscle weakness
- Insomnia
Causes of Imbalance
- Primary hypothyroidism — Hashimoto’s thyroiditis
- Thyroid surgery or radioiodine treatment
- Pituitary dysfunction (secondary hypothyroidism)
- Iodine deficiency
- Certain medications (amiodarone, lithium, carbamazepine)
- Graves’ disease
- Toxic multinodular goitre
- Thyroid adenoma
- Over-replacement with levothyroxine
- Subacute or postpartum thyroiditis
FAQs
Total T4 includes protein-bound hormone that cannot enter cells and is inactive. Free T4 measures only the unbound, biologically available fraction. This makes Free T4 more accurate in people who are pregnant, taking oestrogen, or have liver disease — conditions that alter binding proteins without changing thyroid function.
Yes, this can occur in secondary (central) hypothyroidism, where pituitary failure means insufficient TSH is produced to stimulate the thyroid. Without adequate TSH, the thyroid makes less T4 despite the pituitary signal appearing normal or low. This is why both TSH and Free T4 are measured together.
Free T4 typically rises within 2–4 weeks of starting or adjusting levothyroxine. TSH takes longer — 6–8 weeks — to fully reflect a dose change. Repeat testing is recommended 6–8 weeks after any adjustment.
Free T4 shows modest diurnal variation, peaking in the morning. The variation is small enough that it rarely affects clinical decisions, but testing at a consistent time of day is advisable when monitoring thyroid replacement therapy.
Low Free T4 alongside low TSH suggests secondary or central hypothyroidism — a problem with the pituitary rather than the thyroid itself. This is uncommon and requires investigation for pituitary causes such as a tumour, inflammation, or infiltration.
References
- Garber JR, et al. Clinical practice guidelines for hypothyroidism in adults. Thyroid. 2012;22(12):1200–1235. View source
- Spencer CA. Thyroid function tests — Free T4. Thyroid. 2004;14(Suppl 1):S31–S39. View source
- Peeters RP. Subclinical hypothyroidism. N Engl J Med. 2017;376(26):2556–2565. View source