T3 vs. T4: What the Difference Between Thyroid Hormones Means for You

How does the thyroid work?

The thyroid is a small, butterfly-shaped gland at the base of your neck. Its hormones set the pace of your metabolism, influencing energy, body temperature, heart rate, digestion, mood, the menstrual cycle and bone turnover.

It works on a feedback loop with the pituitary gland in the brain. When the pituitary senses too little thyroid hormone, it releases more TSH (thyroid-stimulating hormone) to push the thyroid harder. When there's plenty, TSH falls. The American Thyroid Association compares this to a thermostat and a heater (ATA). That's why TSH is so useful: it's the body's own reading of whether thyroid hormone levels are right.

What is T4 (thyroxine)?

T4 is the main hormone the thyroid releases, named for the four iodine atoms it carries. It circulates in large amounts and lasts about a week in the blood, giving the body a steady reservoir. T4 has relatively little direct activity; its main job is to be converted into T3 where and when it's needed.

What is T3 (triiodothyronine)?

T3 has three iodine atoms and is the more active hormone, binding to receptors inside cells to switch on metabolic activity. The thyroid makes some T3 directly, but most of the body's T3 comes from converting T4, mainly in the liver and in tissues where T3 acts, such as the brain (ATA). T3 is cleared much faster than T4, lasting about a day.

T3 vs. T4 at a glance

What are "free" and "total" hormone levels?

Almost all T4 and T3 in the blood is attached to carrier proteins. Only the small unbound, or "free," portion can enter tissues. A total T4 or T3 test measures both bound and free hormone, so it shifts when carrier protein levels change. Estrogen, from pregnancy, birth control pills or oral hormone therapy, raises those carrier proteins and therefore raises total T4 and T3. In those situations, TSH and free T4 are the reliable tests (ATA).

Which thyroid tests do you actually need?

• TSH is the first and most sensitive screening test (MedlinePlus). High TSH usually means an underactive thyroid; low TSH usually means an overactive one.
• Free T4 is added when TSH is abnormal, or when a pituitary problem is suspected, to confirm and gauge severity.
• T3 is most useful when hyperthyroidism is suspected, because some people have only T3 elevated early on. It rarely helps in hypothyroidism, since it's often the last test to become abnormal. The ATA notes free T3 measurement is often unreliable (ATA).
• Thyroid antibodies (TPO and thyroglobulin antibodies) help identify Hashimoto's disease, the most common cause of an underactive thyroid (NIDDK). TSH receptor antibodies help identify Graves' disease.
• Reverse T3 is an inactive form of T3. The ATA states that in healthy, non-hospitalized people it does not help determine whether hypothyroidism exists and is not clinically useful.

If you take biotin, tell your clinician before testing. High-dose biotin can interfere with some thyroid assays and produce misleading results.

"My labs are normal, but I still feel awful"

This is a common and frustrating situation, and the feeling is real. It isn't usually explained by a hidden T4-to-T3 "conversion problem," a claim that's popular online but not supported as a common cause of symptoms in people with normal TSH. More often, the explanation is one of the many other conditions that cause fatigue, weight change and brain fog:

• Perimenopause and menopause, especially night sweats and broken sleep
• Iron deficiency or anemia, often from heavy periods
• Depression or anxiety
• Sleep apnea
• Vitamin B12 deficiency, diabetes or medication side effects

A normal TSH is good evidence your thyroid is working adequately at that point in time. If symptoms persist, ask your clinician to look for these other causes, and to repeat thyroid tests if things change. For midlife women, our guides to thyroid and menopause, menopause fatigue and menopause brain fog walk through the overlap.

Do stress and diet affect T3 and T4?

Major physical stress, such as serious illness, surgery or starvation, lowers T3 and raises reverse T3 temporarily. This is a normal adaptive response, and it isn't treated with thyroid hormone. Everyday psychological stress has little measurable effect on thyroid tests (see how stress affects hormones).

How is an underactive thyroid treated?

Thyroid conditions are diagnosed and treated by your primary care clinician or an endocrinologist. For hypothyroidism, the American Thyroid Association recommends levothyroxine (synthetic T4) as the standard of care, with the dose adjusted to keep TSH in range (Jonklaas 2014). The body converts it to T3 as needed, and it's effective for most people.

The same guideline found insufficient evidence to recommend routine combination T4/T3 therapy, and does not recommend desiccated thyroid extract over levothyroxine as standard treatment. Some people with persistent symptoms despite a normal TSH on levothyroxine try adding liothyronine (T3) under specialist care. Because T3 acts quickly and too much can cause a racing heart, anxiety, bone loss and heart rhythm problems, it requires careful dosing and monitoring.

If your thyroid is normal and menopause symptoms remain

Amie does not diagnose or treat thyroid conditions, and Amie HRT+ is not a thyroid treatment. But if your thyroid tests are normal, or your thyroid condition is already well managed, and you still have hot flashes, night sweats, poor sleep or other menopause symptoms, those can be treated in their own right. Hormone therapy is the most effective treatment for hot flashes and night sweats, and non-hormone options are available too.

Amie HRT+ is a compounded estradiol and progesterone cream prescribed after a licensed clinician reviews your health history, including any thyroid condition and medications. It is not FDA-approved, and your thyroid care stays with your own clinician.

The bottom line