T3

Also seen on reports as: Total T3, Free T3, FT3, Liothyronine (in blood)

What is T3?

T3 (Triiodothyronine) is one of the two main hormones the thyroid gland releases — the other being T4 (Thyroxine). T3 is the more biologically active of the two; most T3 in the body is actually made when T4 is converted to T3 in the liver, kidneys, and other tissues, rather than coming directly from the thyroid.

Lab reports usually show one of two forms — Total T3 (all the T3 in the blood, including the portion bound to proteins) or Free T3 (the small fraction that is unbound and biologically available). Free T3 is generally considered the more reliable indicator of thyroid activity in symptomatic patients.

The test is run on a small blood sample drawn from a vein. NABL-accredited Indian labs typically report Total T3 in ng/dL or ng/mL and Free T3 in pg/mL.

Why doctors order it

T3 is rarely ordered on its own as a first-line thyroid test — TSH is. Doctors usually order T3 when TSH is abnormal and they want a fuller picture, or when symptoms strongly suggest hyperthyroidism (an overactive thyroid) even with a borderline TSH.

It is also useful in a specific pattern called 'T3 toxicosis', where T3 is high but T4 looks relatively normal — more common in early hyperthyroidism, particularly in Graves' disease and toxic nodules, both of which are seen in Indian practice.

T3 is sometimes monitored in people taking certain thyroid medications (particularly combination T3/T4 preparations or pure T3 preparations) to check that the dose is in range.

What the result means

A normal TSH with a normal T3 generally means thyroid function is in range. A low TSH paired with a high T3 (and/or high T4) points to hyperthyroidism. A high TSH paired with a low T3 (and low T4) points to hypothyroidism — though in early hypothyroidism, T3 often stays in range even as T4 drops, because the body prioritises T3 production.

Free T3 is preferred over Total T3 in most clinical situations because protein-binding changes (caused by pregnancy, oral contraceptives, oestrogen therapy, and some illnesses) can shift Total T3 without changing what is actually biologically active. Always interpret the value against the reference range printed on the same report.

Acute illness and hospital stays can temporarily lower T3 ('low T3 syndrome' or non-thyroidal illness) without an actual thyroid problem. Doctors recognise this pattern and usually do not start treatment based on T3 alone in someone acutely unwell.

Common factors that affect the result

Pregnancy, oral contraceptives, hormone replacement therapy, and some liver conditions all raise the binding proteins in blood that carry T3 — this elevates Total T3 without necessarily affecting how much active hormone the body is using. Free T3 is much less affected by these factors.

Acute illness, surgery, severe burns, prolonged fasting, and significant weight loss can lower T3 — a normal protective response sometimes called 'low T3 syndrome'. This usually resolves as the underlying illness improves, and is not typically treated as a thyroid disorder.

Certain medications affect T3 levels — amiodarone (a heart medication), beta-blockers, steroids, some anti-seizure medications, and biotin supplements. Biotin in particular can produce misleading readings on many T3 assays; most guidelines recommend stopping biotin 48–72 hours before thyroid testing.

When to talk to your doctor

• Your T3 is above the lab's reference range, particularly if TSH is low — this combination usually warrants follow-up.
• Your T3 is high and you have symptoms suggesting an overactive thyroid — palpitations, unexplained weight loss, tremor, heat intolerance, anxiety, frequent bowel movements.
• You are on combination thyroid medication and your T3 has shifted noticeably from previous readings.
• You take biotin or are in the middle of a course of any medication known to affect thyroid binding proteins, and you want to know if the result is reliable.
• You are planning pregnancy, are pregnant, or are post-partum with thyroid symptoms — interpretation differs in these life stages.

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