Thyroid test results explained: TSH, free T4, T3 and antibodies

Here are your thyroid test results explained in one idea: TSH is the signal, free T4 is the hormone. TSH tells you how hard your body is pushing the thyroid. Free T4 tells you how much hormone the thyroid is actually putting out. Read the two together and most reports stop looking like a puzzle.

This guide walks through each line on a typical thyroid report, the patterns doctors look for, and the questions worth bringing to your next appointment. It won't diagnose you. It will help you follow the conversation.

What a thyroid blood test usually includes

A basic check is often just TSH. Many labs only add more tests when TSH is outside the range: T4 if TSH is high, and T4 plus T3 if TSH is low. A fuller panel can include:

• TSH (thyroid-stimulating hormone)
• Free T4 or total T4 (thyroxine)
• T3 or free T3 (triiodothyronine)
• Thyroid antibodies, such as TPO antibodies, thyroglobulin antibodies and TSH receptor antibodies

You don't need all of these every time. Which ones your doctor orders depends on what they're trying to find out.

TSH: the signal from the pituitary

TSH is made by the pituitary gland, not the thyroid. Think of it as a volume knob. When thyroid hormone runs low, TSH goes up to push the thyroid harder. When there's too much thyroid hormone, TSH drops.

That's why the direction can feel backwards:

• High TSH most often points to an underactive thyroid (hypothyroidism).
• Low TSH usually points to an overactive thyroid (hyperthyroidism), or to taking more thyroid hormone than you need.

The American Thyroid Association calls TSH the best first test, because it often shifts before T4 or T3 do.

Free T4: the hormone itself

T4 is the main hormone the thyroid releases. Most of it travels through the blood attached to proteins. Total T4 counts the attached part and the free part. Free T4 counts only the unbound hormone that can reach your tissues, so doctors usually prefer it.

This matters because some things raise those carrier proteins without any thyroid problem. Pregnancy and birth control pills that contain estrogen can push total T4 up while free T4 stays normal.

T3: the active form

Your liver and kidneys turn most T4 into T3, the form your cells actually use. T3 is mainly checked when doctors suspect an overactive thyroid, or to see how strong it is. It's rarely helpful for an underactive thyroid, because T3 is usually the last value to fall. Someone can have a clearly underactive thyroid and still have a normal T3.

The TSH normal range, and why yours may differ

There is no single "normal TSH". Each lab sets its own range based on its equipment and methods. Here is how much published adult ranges differ, even between trusted sources:

Test	British Thyroid Foundation	Cleveland Clinic panel	MedlinePlus
TSH	0.4 to 4.0 mU/L	0.3 to 4.0 mIU/L	about 0.4 to 4.8 µU/mL
Free T4	9 to 25 pmol/L	0.7 to 2.1 ng/dL	not listed
Free T3	3.5 to 7.8 pmol/L	2.3 to 4.1 pg/mL	not listed

A few things to notice:

• Units differ. mU/L, mIU/L and µU/mL describe the same TSH amount, so those numbers line up. Free T4 in pmol/L and ng/dL do not, so never compare them directly.
• Age matters. Some labs use a higher TSH upper limit for older adults, up to around 7. Children and teenagers have their own free T4 ranges.
• Pregnancy changes the range. TSH ranges in pregnancy differ from those for everyone else, so a result may be read differently while you're pregnant.
• Whether you're male or female can shift some ranges too. Cleveland Clinic, for example, lists different free T4 ranges for teenage boys and girls.

The rule is simple: use the range printed on your own report, and ask your doctor what it means for you.

High TSH, low T4 and other common patterns

Doctors read TSH and free T4 as a pair. These are the patterns you'll see most often:

TSH	Free T4	What it often points to
High	Low	An underactive thyroid (hypothyroidism)
High	Normal	Subclinical hypothyroidism
Low	High	An overactive thyroid (hyperthyroidism)
Low	Normal, with high T3	Sometimes an overactive thyroid that shows up in T3 first
Low	Low	A pituitary problem, or the effect of a serious illness elsewhere
Normal	Normal	The thyroid is usually working as it should

"Often points to" is doing real work in that table. Your doctor also weighs your symptoms, medical history, medicines and sometimes a repeat test before naming anything.

Subclinical hypothyroidism

This is the label for a TSH that's raised while free T4 is still normal. It's common, and it often causes no symptoms at all. When symptoms do show up, they can include tiredness, feeling cold, constipation, dry skin or heavier periods.

Cleveland Clinic splits it into two grades: a TSH of about 4.5 to 9.9, and a TSH of 10 or more. A few things are worth knowing:

• Many cases settle on their own. Doctors often repeat TSH within about three months before deciding anything.
• Some cases progress. Around 2% to 6% of people with it develop a fully underactive thyroid each year.
• Treatment is a judgement call. It's more likely to be offered when TSH is 10 or higher, when someone younger has symptoms, or for people trying to get pregnant.

What thyroid antibodies tell you

Antibody tests don't measure how well your thyroid works. They help explain why it might not be working normally.

• TPO antibodies are the main marker for Hashimoto's disease, a common cause of an underactive thyroid. A positive result with a normal TSH may mean the thyroid slows down later.
• Thyroglobulin antibodies can also be raised in Hashimoto's, and are tracked after thyroid cancer treatment.
• TSH receptor antibodies (TRAb), including thyroid stimulating immunoglobulin (TSI), are linked to Graves' disease. About 95% of people with Graves' have raised TRAb.

Two calm facts help here. Some people test positive without having any thyroid disease. And for adults, repeating TPO antibody tests is rarely useful, because TSH and free T4 are what show how the thyroid is actually doing.

Things that can throw your results off

Before you worry about a borderline number, check whether any of these apply:

• Biotin supplements. Biotin (vitamin B7), common in hair and nail products, can make thyroid tests look abnormal when they're not. Advice on how long to pause it varies, from about two days (American Thyroid Association) to three to five days (Cleveland Clinic). Ask whoever ordered your test.
• Medicines. Steroids, lithium, amiodarone, some seizure medicines and birth control pills can all affect results. Tell your doctor everything you take, and don't stop a prescribed medicine on your own.
• Recent illness. A serious infection, a heart attack, or major liver or kidney disease can shift thyroid levels for a while.
• Time of day. TSH changes through the day, so MedlinePlus suggests early morning tests. When you repeat a test, try to go at a similar time.
• Iodine. Iodine supplements, or the contrast dye used in some CT scans, can affect the thyroid.

Questions to ask about your thyroid results

1. Which range does my lab use, and where does my result sit in it?
2. Should this test be repeated, and when?
3. Could any medicine or supplement I take be affecting it?
4. Do I need free T4, T3 or antibody tests as well?
5. If I'm pregnant or planning to be, does this change anything?
6. What would make you start, stop or change treatment?

If you get thyroid tests more than once, keep your reports together. A single TSH is a snapshot. Several results over months show which direction things are moving.

Saluvo can help with that part. Two AI models read each report you upload, you confirm any value they disagree on, and the app shows how your TSH and T4 change across reports. It's there to help you prepare, not to replace your doctor's reading.