T3 Thyroid Hormone: Testing, Conversion, and When Supplementation Helps
T3 vs. T4: The Two Main Thyroid Hormones
The thyroid gland produces two active hormones: thyroxine (T4) and triiodothyronine (T3). The numbers refer to how many iodine atoms are attached to each molecule — four for T4, three for T3.
Of the two, T3 is the more biologically active form. It enters cells directly and drives metabolic processes: regulating how quickly your heart beats, how fast you burn calories, body temperature, mood, cognitive function, and much more. T4, by contrast, is largely a storage or transport form — it circulates in the bloodstream but must be converted to T3 to have its effect in tissues.
The thyroid gland produces about 80% T4 and 20% T3 directly. The remainder of your body's T3 comes from conversion of T4 in peripheral tissues, primarily the liver, kidneys, and muscle.
Key point: Most standard thyroid treatment (levothyroxine / Synthroid) replaces only T4. The body is then expected to convert T4 into T3 as needed. For most people this works well — but not for everyone.
T4 to T3 Conversion: When It Works and When It Doesn't
The conversion of T4 to T3 is carried out by enzymes called deiodinases. These enzymes remove one iodine atom from T4, producing active T3. Several factors can impair this conversion:
- Selenium deficiency — deiodinase enzymes are selenium-dependent
- Chronic illness or major surgery — the body diverts conversion away from T3 as a protective response
- Chronic stress and high cortisol — cortisol inhibits deiodinase activity
- Caloric restriction or severe dieting
- Liver or kidney disease — major sites of conversion
- Genetic variants in deiodinase genes (DIO1, DIO2) — some people are genetically poor converters of T4 to T3
- Older age — conversion efficiency declines
When conversion is impaired, a patient's TSH and T4 levels can appear normal while T3 remains low — creating a situation where standard labs look fine but symptoms persist.
Free T3 Testing: What It Measures and When to Order It
Thyroid hormones circulate in two forms: bound to proteins (inactive) and free (active). The free T3 (fT3) test measures only the unbound, biologically active fraction — making it more clinically meaningful than total T3.
Normal Free T3 Range
Reference ranges vary slightly by laboratory, but a typical free T3 range is approximately 2.3–4.1 pg/mL (or 3.5–6.5 pmol/L in international units). Many endocrinologists aim for the mid-to-upper half of the range in treated hypothyroid patients, not just "technically normal."
When to Test Free T3
- Patients on levothyroxine who remain symptomatic despite a normal TSH
- Evaluation for possible T4-to-T3 conversion problems
- Patients with hyperthyroidism — free T3 is often the first to elevate in Graves' disease and toxic nodules
- Monitoring patients on combination T4/T3 therapy (levothyroxine + liothyronine)
- Evaluation of thyroid status in sick euthyroid syndrome
Note: Free T3 is not routinely needed in every thyroid check-up. TSH and free T4 remain the primary screening tests. Your endocrinologist will determine when free T3 testing adds value for your specific situation.
Reverse T3: What It Is and What It Means
T4 can be converted in two directions. The normal pathway produces active T3. The alternate pathway produces reverse T3 (rT3) — a structurally similar molecule that is biologically inactive and actually competes with T3 at receptor sites, potentially blocking its effects.
The body increases rT3 production deliberately during physiological stress — illness, major surgery, starvation, or prolonged intense stress. This is thought to be a protective mechanism to slow metabolism when energy conservation is needed. It is known as euthyroid sick syndrome or non-thyroidal illness syndrome.
Is High Reverse T3 a Problem?
This is an area of ongoing debate in endocrinology. Some integrative and functional medicine practitioners attribute a range of symptoms to elevated rT3 and advocate for treatment, while most academic endocrinologists consider elevated rT3 in sick patients to be a normal adaptive response that should not be treated directly.
In a healthy person not under acute illness or stress, a persistently elevated rT3 with low free T3 may warrant discussion with your endocrinologist. Routine rT3 testing is not recommended by current major guidelines (ATA, AACE) for most patients.
Symptoms of Low Free T3
Even when TSH and T4 are normal, a low or low-normal free T3 may contribute to:
- Persistent fatigue and low energy despite adequate sleep
- Brain fog, difficulty concentrating, memory issues
- Depression or low mood
- Cold intolerance
- Constipation
- Dry skin, hair thinning, or hair loss
- Difficulty losing weight despite diet and exercise
- Slow heart rate or low blood pressure
These symptoms overlap significantly with hypothyroidism but can persist in patients whose TSH has been normalized on levothyroxine. This is sometimes called persistent hypothyroid symptoms on levothyroxine — a real and recognized clinical phenomenon.
Liothyronine (Cytomel): T3 Replacement Therapy
Liothyronine (brand name Cytomel) is synthetic T3. It can be added to levothyroxine therapy for patients who continue to have symptoms despite optimal T4 dosing and a normal TSH.
How It Works
Unlike levothyroxine, liothyronine does not need to be converted — it is immediately bioavailable. Because it has a shorter half-life (about 24 hours versus 7 days for levothyroxine), it causes more fluctuation in T3 levels throughout the day, which can occasionally cause palpitations or anxiety, especially at higher doses.
Who May Benefit
- Patients with persistent symptoms on levothyroxine with consistently low free T3 despite normal TSH
- Patients with genetic deiodinase variants (DIO2 polymorphism) identified on testing
- Some patients after total thyroidectomy (who have lost all native T3 production)
Who Should Not Use Liothyronine
- Patients with known heart disease or arrhythmias — the sharper peaks of T3 can stress the heart
- Patients with uncontrolled osteoporosis — excess thyroid hormone accelerates bone loss
- Older patients in whom TSH suppression carries greater cardiovascular risk
Combination T4/T3 Therapy: What the Evidence Shows
- The 2014 ATA guidelines acknowledge that a subset of patients do not feel well on T4 alone and may benefit from combination therapy
- Most randomized trials show no difference on average — but some patients clearly do better on combination therapy
- A 2019 clinical practice guideline recommends individualized decision-making, with careful monitoring
- Desiccated thyroid extract (Armour Thyroid) contains both T4 and T3 in a fixed ratio and is another option some patients prefer
Still Symptomatic on Levothyroxine? What to Do
If you are taking levothyroxine correctly (see our thyroid medication handout) and your TSH is in the optimal range but you still feel unwell, ask your endocrinologist to:
- Check a free T3 level to see if conversion is adequate
- Review your levothyroxine absorption (are you taking it properly, avoiding interfering supplements?)
- Ensure your TSH is optimal for you — not just "in range" but in the range where you feel well
- Consider whether other conditions are contributing (adrenal fatigue is a common mimicker)
- Discuss whether a trial of combination T4/T3 therapy is appropriate for your situation
Important: Do not self-supplement T3 products purchased online. Over-the-counter "thyroid support" supplements vary widely in content and can cause dangerous thyroid suppression or toxicity. Any T3 therapy should be monitored by an endocrinologist with regular blood tests.