T4 to T3 Conversion Calculator: How Well Do You Convert?
Slide your labs into place and watch what your body does with its T4:
Different units on your report? Free T3 in pmol/L divided by 1.536 gives pg/mL. Free T4 in pmol/L divided by 12.87 gives ng/dL.
Written and medically reviewed by Dr. Westin Childs, D.O. Last reviewed: August 28, 2026
How To Use This Calculator:
#1. Get Your Latest Labs
You need free T3 and free T4 at minimum. Add total T3 and reverse T3 if you have them, and the score gets more accurate. If your doctor only ordered TSH, that by itself tells you something: ask for the full panel next time.
#2. Enter Your Numbers
Type them exactly as they appear on your report, in the units shown. If your lab uses different units, the tool tells you how to convert.
#3. Read Your Conversion Score
You’ll get a 0 to 100 score for how well your body turns T4 into active T3, plus each ratio explained: what it is, where yours falls, and what tends to push it there.
#4. Save It And Track It
Your labs go on your free Thyroid Tracker, so when you fix your conversion, you’ll have the before and after.
Why Conversion Is Where Thyroid Treatment Fails
T4 is a storage hormone. It does almost nothing until your body converts it into T3, the active hormone that your cells actually use.
And that conversion step is exactly where things go wrong for a huge number of thyroid patients. Your thyroid (or your levothyroxine) can supply plenty of T4 while your body quietly fails to turn it into enough T3.[1]
Here’s why your doctor misses it: TSH and free T4 both look fine in that situation. The problem only shows up in free T3 and reverse T3, the two labs least likely to get ordered.
What slows conversion down? The usual suspects: chronic stress and high cortisol, inflammation, dieting too hard for too long, and missing the nutrients the conversion enzymes need, mainly selenium and zinc.[2] Illness pushes the same direction, which is why hospitals see low T3 constantly.[3]
The reverse T3 piece matters because your body can also shunt T4 into reverse T3, an inactive dead end that rises when you’re stressed, sick, or under-eating. A high reverse T3 with a low free T3 is the signature of a conversion problem.
The Ratios, Explained
- Free T3 to free T4 ratio: the direct read on conversion. Plenty of T4 with proportionally little T3 means the raw material is there and the conversion isn’t.
- Free T3 to reverse T3 ratio: the activity read, on the same scale as my Reverse T3 Ratio Calculator: free T3 in pg/mL times 100, divided by reverse T3 in ng/dL. A ratio of 20 or higher means no reverse T3 pooling, 10 to 19 suggests thyroid pooling, and under 10 is reverse T3 dominance.
- Total T3 to reverse T3 ratio: the backup read when you have total T3 on your report. Above about 10 is the usual target.
No single ratio is a diagnosis. But when two or three of them point the same direction and you still have symptoms, you have a real answer for why you feel the way you do, and something concrete to bring to your doctor.
Frequently Asked Questions
With free T3 in pg/mL and free T4 in ng/dL, most people who feel well land around 3.0 or higher. Below about 2.5, and especially below 2.0, the pattern starts looking like a conversion problem: enough raw material, not enough active hormone. Where exactly you feel best is individual, which is why tracking your own ratio over time beats any single cutoff.
Because most doctors treat to TSH, and TSH mostly reflects T4. Your TSH and free T4 can both sit in range while your free T3 sits at the bottom and your reverse T3 sits at the top. Nothing on a TSH-only panel catches that. Bring the actual free T3 and reverse T3 numbers and ask about them directly.
Stress, illness, inflammation, aggressive dieting, and certain medications like beta blockers and steroids. Your body makes reverse T3 on purpose to slow metabolism during hard times. The problem is when the hard times never end and the shunting becomes your baseline.
Work the list that slows it down: get the stress and sleep handled, stop the crash dieting, treat the inflammation, and cover selenium and zinc, the minerals the conversion enzymes are made from. Re-test in 8 to 12 weeks and watch your ratios, because that’s the scoreboard.
Related Tools
- Optimal Thyroid Lab Calculator: every lab, not just the ratios
- Thyroid Score Test: how optimized is your thyroid overall
- All Thyroid Calculators & Quizzes: the full set of free tools
Scientific References
- Bianco AC, Kim BW. Deiodinases: implications of the local control of thyroid hormone action. J Clin Invest. 2006;116(10):2571-2579.
- Ventura M, Melo M, Carrilho F. Selenium and thyroid disease: from pathophysiology to treatment. Int J Endocrinol. 2017;2017:1297658.
- Fliers E, Bianco AC, Langouche L, Boelen A. Thyroid function in critically ill patients. Lancet Diabetes Endocrinol. 2015;3(10):816-825.