Free T3 and Free T4: What Your Full Thyroid Panel Means

The Thyroid Hormone Cascade

The hypothalamus releases thyrotropin-releasing hormone (TRH), which signals the pituitary to release TSH (thyroid-stimulating hormone). TSH signals the thyroid gland to produce thyroxine (T4). Most T4 is converted to the active form, triiodothyronine (T3), in peripheral tissues — especially the liver, kidneys, and muscles. T3 is 3–5 times more biologically potent than T4. TSH is an indirect measure of thyroid status — it reflects what the pituitary thinks is happening, not what the tissues are actually receiving.

Full Thyroid Panel Reference Ranges

Test	Normal Range	Research Optimal
TSH	0.5–4.5 mIU/L	1.0–2.5 mIU/L
Free T4 (FT4)	0.8–1.8 ng/dL	1.1–1.7 ng/dL
Free T3 (FT3)	2.3–4.2 pg/mL	3.0–4.2 pg/mL
Reverse T3 (rT3)	9.2–24.1 ng/dL	Below 15 ng/dL
TPO antibodies (anti-TPO)	Below 35 IU/mL	Negative

Reading the Pattern: What Different Combinations Mean

High TSH + low FT4: Primary hypothyroidism — the thyroid is underperforming. The pituitary is producing more TSH to try to stimulate it. This is the most common pattern in Hashimoto's thyroiditis and iodine deficiency.

Low TSH + high FT4 + high FT3: Primary hyperthyroidism — the thyroid is overproducing hormones. Common causes are Graves' disease (autoimmune hyperthyroidism) and toxic nodular goitre.

Normal TSH + normal FT4 + low FT3: Conversion problem — the thyroid produces normal T4, but peripheral conversion to active T3 is impaired. This is common in chronic illness, aggressive caloric restriction, selenium deficiency, high cortisol states, and some cases of Hashimoto's where patients feel unwell despite "normal" TSH. This pattern is missed if only TSH is measured.

High TSH + normal FT4: Subclinical hypothyroidism — the thyroid is being pushed hard to maintain normal hormone output. Whether to treat is debated; most guidelines recommend treating if TSH is consistently above 10 mIU/L or if the patient has symptoms.

When to Add Thyroid Antibodies

Anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin (anti-TG) antibodies confirm autoimmune thyroid disease (Hashimoto's or Graves'). High anti-TPO antibodies with a normal TSH in a symptomatic patient confirm Hashimoto's thyroiditis — the thyroid may still produce adequate hormones, but the autoimmune process is active and TSH should be monitored closely over time. Testing antibodies is particularly useful when TSH is borderline and a cause is needed.

Why "Normal TSH" Doesn't Always Mean Normal Thyroid

TSH has a wide reference range (0.5–4.5 mIU/L) that encompasses a roughly 9-fold difference in signalling intensity. An individual's personal TSH set point is genetically determined and can be quite narrow — someone who normally runs at 1.0 and now sits at 4.0 may feel profoundly hypothyroid while technically "in range." Free T3 and Free T4 provide additional objective data when symptoms and TSH don't align.