T4 and T3: The Difference Between the Thyroid Hormones

T4 is the hormone the thyroid makes the most of, but it works as a reserve: the one doing the work inside your cells is T3. Between the two there's a step called conversion — and that is exactly where many of the symptoms that persist despite "normal" labs come from.

Few things cause as much confusion in the exam room as the alphabet soup of a thyroid panel. The patient arrives with the printout, sees free T4, free T3, reverse T3, and no one has ever explained what each one means — or why two of them can be within the reference range while the fatigue, the cold and the brain fog stay exactly the same.

After 25 years of caring for patients with thyroid dysfunction, I can say that understanding the difference between T4 and T3 is what moves a patient from simply receiving a lab report to truly understanding their own treatment. Every claim in this article comes with the scientific reference that supports it; the full list is at the end.

T4: the storage hormone

T4 (thyroxine) accounts for most of what the thyroid produces. It circulates in the blood in large amounts, has a long half-life and works as a strategic reserve: it's available, but not yet ready to act (Chaker et al., Lancet, 2017).

T4 has very little metabolic activity. It doesn't speed up your metabolism, doesn't warm your body, doesn't sustain your cells' energy. To do any of those things, it first has to be converted.

T3: the hormone that does the real work

T3 (triiodothyronine) is the active form. It's the one that enters the cell, binds to the receptor in the nucleus and flips the metabolic switch: energy production, body temperature, heart rate, bowel function, hair growth, mental clarity, mood.

T3 is several times more potent than T4, but the thyroid produces only a small portion of it directly. Most of the T3 circulating in the body isn't made in the gland: it's manufactured outside of it, from T4 (Bianco and Kim, Journal of Clinical Investigation, 2006).

If T4 is the raw material, T3 is the finished product. And a factory with a full warehouse produces nothing if the assembly line has stopped.

Conversion: the step almost no one investigates

The conversion of T4 into T3 happens mainly in the liver, the kidneys and other tissues, through enzymes called deiodinases. These enzymes are selenoproteins: they depend on selenium to exist and function (Bianco et al., Endocrine Reviews, 2002).

This has an important clinical consequence: the thyroid may be producing enough T4, the free T4 test may come back looking great, and there can still be too little active hormone inside the cell, because conversion isn't happening well.

That's why there are patients with a normal free T4 and a free T3 dragging along the bottom third of the reference range — with every classic symptom of hypothyroidism and the message that "everything is normal."

What commonly blocks conversion

In clinical practice, the factors that most often interfere with this step are:

• Selenium, iron or iodine deficiency — a shortage of these nutrients impairs the production and metabolism of thyroid hormones (Zimmermann and Köhrle, Thyroid, 2002)
• Zinc deficiency, since zinc helps regulate the conversion enzymes themselves (Severo et al., International Journal for Vitamin and Nutrition Research, 2019)
• Chronic stress, acute illness, persistent inflammation and prolonged fasting — situations in which the body reduces production of active T3 (Fliers et al., Lancet Diabetes & Endocrinology, 2015)
• Very restrictive diets and prolonged calorie restriction, which the body interprets as scarcity
• Insulin resistance and other metabolic changes
• An overburdened liver and imbalanced gut flora, since much of the conversion depends on how well these organs are working
• Some commonly used medications, which should be evaluated case by case by the treating physician

Reverse T3: the parking brake that's been pulled

There's a third pathway that's rarely explained. The body can also turn T4 into reverse T3 — an inactive form that does not switch on the cell's metabolism.

This mechanism isn't a defect: it's a protection. In the face of intense stress, prolonged fasting, serious illness or inflammation, the body diverts production toward the inactive form and saves energy. This pattern is well described in the literature as euthyroid sick syndrome, with a drop in T3 and a rise in reverse T3 (Fliers et al., Lancet Diabetes & Endocrinology, 2015).

The problem comes when the trigger doesn't go away and that brake stays on for months, in a patient who still has symptoms and whose TSH and free T4 look normal on paper.

In all honesty: measuring reverse T3 is not part of the routine workup in conventional medicine — it's a marker used mainly in functional approaches, as an investigative tool, and it must be interpreted within the clinical context, never on its own.

Why TSH alone can't see this

TSH is the signal the pituitary sends to the thyroid. It tells you whether the gland is being pushed to make more or less hormone — and nothing beyond that.

TSH doesn't measure how well T4 is converted into T3, or what actually reaches and acts inside the cell. Assessing the thyroid by TSH alone is like checking the order placed with the factory without ever once looking at what came off the line.

If you take levothyroxine and still have symptoms

Levothyroxine is T4 replacement — in other words, it delivers raw material to the body and relies on conversion for the rest. It is, and remains, the standard treatment for hypothyroidism, with well-established effectiveness and safety (Jonklaas et al., Thyroid, 2014). In most patients, it works well.

But there is a group for whom the story doesn't add up, and this is documented in the literature:

• In a study of patients without a functioning thyroid who were taking levothyroxine, about 15% kept their free T3 below the levels seen in healthy people, and more than 20% did not have free T3 or free T4 within the reference interval — despite a normal TSH (Gullo et al., PLoS One, 2011).
• In an international survey of 12,146 patients being treated for hypothyroidism, satisfaction with treatment was low, with a median of 5 on a 0-to-10 scale among those taking levothyroxine alone; complaints about weight, energy, mood and memory were common (Peterson et al., Thyroid, 2018).

In other words: a normal TSH is not the same as a patient restored to health, and the lingering discomfort isn't the patient's imagination — it's a problem recognized by the medical literature itself.

In these cases, the answer isn't guessing at doses or switching medications on your own. It's investigating why conversion isn't happening well and correcting the cause: nutrients, inflammation, gut, stress, sleep, metabolism.

As for replacement that combines T4 and T3, the current scientific position is one of caution: the consensus document from the American, British and European thyroid associations reviewed 14 clinical trials and found no consistent benefit of the combination over levothyroxine alone, while acknowledging that it may be considered, on an individualized and monitored basis, in selected patients who remain symptomatic (Jonklaas et al., Thyroid, 2021). It's a medical decision, made case by case — not a protocol for everyone.

Never adjust, stop or switch your thyroid medication on your own, and never based on an article. Thyroid hormone at the wrong dose affects the heart and the bones.

What to ask for at your next appointment

An evaluation that looks at this step usually includes high-sensitivity TSH, free T4, free T3, reverse T3, anti-TPO and anti-TG. The last two look for autoimmunity — Hashimoto's thyroiditis is the most common cause of hypothyroidism in regions with adequate iodine intake (Chaker et al., Lancet, 2017).

And there's a detail that changes how everything is read: being within the reference range isn't enough. A reference range is a population statistic, not your personal point of balance. A free T3 scraping the lower limit, with obvious symptoms, deserves investigation — not an "it's normal."

At the Dr. André Azevedo Clinic in Campinas, we evaluate the complete thyroid panel, the peripheral conversion of T4 into T3, the nutrients involved in that step and the factors that block it, building an individualized protocol. If your labs came back normal but your symptoms haven't gone away, there is something to investigate.

Scientific references

1. Bianco AC, Salvatore D, Gereben B, Berry MJ, Larsen PR. Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases. Endocrine Reviews. 2002;23(1):38-89. PMID: 11844744.
2. Bianco AC, Kim BW. Deiodinases: implications of the local control of thyroid hormone action. Journal of Clinical Investigation. 2006;116(10):2571-2579. PMID: 17016550.
3. Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet. 2017;390(10101):1550-1562. PMID: 28336049.
4. Gullo D, Latina A, Frasca F, Le Moli R, Pellegriti G, Vigneri R. Levothyroxine monotherapy cannot guarantee euthyroidism in all athyreotic patients. PLoS One. 2011;6(8):e22552. PMID: 21829633.
5. Peterson SJ, Cappola AR, Castro MR, et al. An online survey of hypothyroid patients demonstrates prominent dissatisfaction. Thyroid. 2018;28(6):707-721. PMID: 29620972.
6. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670-1751. PMID: 25266247.
7. Jonklaas J, Bianco AC, Cappola AR, et al. Evidence-based use of levothyroxine/liothyronine combinations in treating hypothyroidism: a consensus document. Thyroid. 2021;31(2):156-182. PMID: 33276704.
8. Fliers E, Bianco AC, Langouche L, Boelen A. Thyroid function in critically ill patients. Lancet Diabetes & Endocrinology. 2015;3(10):816-825. PMID: 26071885.
9. Zimmermann MB, Köhrle J. The impact of iron and selenium deficiencies on iodine and thyroid metabolism: biochemistry and relevance to public health. Thyroid. 2002;12(10):867-878. PMID: 12487769.
10. Severo JS, Morais JBS, de Freitas TEC, et al. The role of zinc in thyroid hormones metabolism. International Journal for Vitamin and Nutrition Research. 2019;89(1-2):80-88. PMID: 30982439.