Thyroid Freedom · For Patients

Reading your thyroid labs: TSH, free T4, free T3, reverse T3, and antibodies.

A thyroid panel is five or six numbers, and most patients are told only one of them. TSH is a useful screening test, but on its own it cannot tell you whether you are converting T4 to T3, whether your disease is autoimmune, or why you still feel unwell with a “normal” result. This guide explains what each test measures, what the ranges mean, and which numbers matter most for each kind of treatment.

By Brittany Henderson, MD, board-certified endocrinologist · October 2026

The tests, one at a time

Test	What it measures	Typical reference range	What a high value suggests	What a low value suggests
TSH (thyroid-stimulating hormone)	The pituitary's signal to the thyroid; rises when the body senses too little thyroid hormone	About 0.4 to 4.0 mIU/L; many clinicians aim for 0.5 to 2.5 on treatment	Under-replaced or untreated hypothyroidism	Over-replaced, or hyperthyroidism
Free T4	The unbound storage hormone available to tissues	About 0.8 to 1.8 ng/dL	Over-replacement with T4; hyperthyroidism	Under-replacement; untreated hypothyroidism
Free T3	The unbound active hormone	About 2.3 to 4.2 pg/mL	Over-replacement with T3 or DTE; hyperthyroidism	Poor T4-to-T3 conversion; illness; calorie restriction
Reverse T3	An inactive form the body makes from T4 under stress or illness	About 10 to 24 ng/dL	Illness, inflammation, fasting, high-dose T4; interpret with free T3	Rarely meaningful
TPO antibodies	Immune attack on thyroid peroxidase	Below about 35 IU/mL, lab-dependent	Hashimoto's thyroiditis; present in most autoimmune hypothyroidism	Autoimmunity less likely
Thyroglobulin antibodies	Immune attack on the thyroglobulin protein	Lab-dependent	Hashimoto's; also tracked after thyroid cancer	Autoimmunity less likely

Reference ranges vary by lab, so always read your results against the range printed on your own report.

Why TSH alone is not enough

TSH is a signal from the pituitary, not a measurement of the hormone your cells are using. It responds mostly to T4, so a person on levothyroxine can have a perfectly normal TSH while their free T3 sits at the bottom of the range. That pattern, normal TSH, high-normal free T4, low-normal free T3, is the signature of a poor converter, and it is exactly the patient the thyroid associations describe as a candidate for a trial of combination therapy. You cannot see it without free T3.

TSH also lags. After a dose change it takes four to six weeks to settle, which is why rechecking sooner can mislead. And TSH is less reliable in pregnancy, in older adults, and in people with pituitary disease, where free hormone levels matter more.

What "normal" means and why the target is personal

A reference range is the middle 95 percent of a population, not a definition of health. A TSH of 3.8 and a TSH of 0.9 are both "normal," but many patients feel different at each. On treatment, clinicians commonly aim for the lower half of the range and adjust by how the patient feels, provided free T4 and free T3 stay in range. The goal is a normal TSH and a patient who feels well, not just the first of those.

How the numbers change on each treatment

On levothyroxine alone, free T4 often sits high-normal and free T3 mid to low. A low free T3 with persistent symptoms is the finding that opens the door to other options.
On combination therapy or DTE, free T4 usually runs lower and free T3 higher. Free T3 peaks two to four hours after a T3 dose, so timing the blood draw matters: draw before the morning dose, or at the same interval after it every time, so results are comparable.
On any T3-containing therapy, a suppressed TSH or an above-range free T3 means too much, regardless of symptoms, because of the heart and bone risks of over-replacement.

Antibodies: why they matter even if you are already diagnosed

Most hypothyroidism in the United States is Hashimoto's thyroiditis, an autoimmune condition. Knowing your antibody status matters because autoimmune patients are more likely to react to fillers and dyes, more likely to have other autoimmune conditions such as celiac disease that affect absorption, and more likely to have fluctuating hormone needs as the gland is gradually damaged. Antibody levels do not need to be rechecked often, but every thyroid patient should know them at least once.

Before you change anything

Fatigue, weight gain, hair loss, and brain fog are not specific to the thyroid. A low ferritin, a low vitamin B12 or D, untreated sleep apnea, perimenopause, or depression can produce the same picture with perfect thyroid labs. Ask for these to be checked alongside the thyroid panel; it is the most common way a "thyroid problem" turns out to be something else that is easier to fix.

Questions to bring to your appointment

Can we run the full panel, including free T3 and antibodies, not just TSH?
Where in the range do my numbers sit, and where do I feel best?
If my free T3 is low, what does that mean for my treatment options?
Have we ruled out iron, B12, vitamin D, and sleep problems?

About the author

Brittany Henderson, MD is a board-certified endocrinologist and Medical Director of The Thyroid Foundation. She is the Medical Doctor at MyThyroidDoctor, a thyroid-only telehealth platform. Follow her on Instagram at @mythyroiddoctor.