TSH and free T4 interpretation: where your results fall on the grid
TSH and free T4 are read as a pair. TSH is the pituitary’s signal to the thyroid, and free T4 is the hormone the thyroid makes in response. Each can be low, in range, or high, which gives nine combinations, and each combination points somewhere different. Enter both results and the ranges printed beside them on your report. The tool marks your cell on the grid.
| Free T4 low | Free T4 in range | Free T4 high | |
|---|---|---|---|
| TSH high | Primary hypothyroidism | Subclinical hypothyroidism | TSH and free T4 both high |
| TSH in range | Low free T4, normal TSH | Both within range | High free T4, normal TSH |
| TSH low | TSH and free T4 both low | Subclinical hyperthyroidism | Primary hyperthyroidism |
How the pair works
When free T4 falls, the pituitary raises TSH to push the thyroid harder. When free T4 rises, TSH drops. So in most thyroid problems the two move in opposite directions, and the problem is in the thyroid itself. That’s the diagonal from top left to bottom right of the grid: high TSH with low free T4 is an underactive thyroid, and low TSH with high free T4 is an overactive one.
TSH responds to free T4 on a roughly logarithmic scale, so a small change in free T4 moves TSH a lot. That’s why TSH often leaves its range first. When TSH is out of range and free T4 is still inside its own, the pattern is called subclinical: subclinical hypothyroidism with a high TSH, subclinical hyperthyroidism with a low one.
The nine patterns
| Free T4 low | Free T4 in range | Free T4 high | |
|---|---|---|---|
| TSH high | Primary hypothyroidism | Subclinical hypothyroidism | Both high: uncommon, check for interference |
| TSH in range | Possible central hypothyroidism, illness, or medication | Normal | Levothyroxine timing, medication, or interference |
| TSH low | Possible central hypothyroidism, illness, or recent treatment | Subclinical hyperthyroidism | Primary hyperthyroidism |
The four corners and the center are the common results. The other cells are where TSH and free T4 don’t move in opposite directions. A review of thyroid test pitfalls by Koulouri and colleagues recommends checking the clinical picture first, then pregnancy, illness, and medications such as levothyroxine, amiodarone, and heparin, and only then the assay itself.
TSH and free T4 both high is unusual. The everyday explanations are a levothyroxine dose taken irregularly, so free T4 is up from recent doses while TSH still reflects the missed ones, or interference in one of the assays. Rarer causes are resistance to thyroid hormone and a pituitary tumor that makes TSH.
Low free T4 with a TSH that isn’t high is the pattern where TSH can’t be trusted. A failing pituitary can’t raise TSH, so it stays normal or low even though the thyroid is underactive. This is central hypothyroidism. The same pattern is common during and after a serious illness, and some medications cause it.
High free T4 with a normal TSH is often levothyroxine taken in the hours before the draw, which briefly raises free T4. Amiodarone and heparin can also raise it.
The ranges matter
TSH ranges vary less than free T4 ranges, but they still vary. The tool starts with 0.45 to 4.5 mIU/L for TSH and 0.8 to 1.8 ng/dL (about 10 to 23 pmol/L) for free T4, which are typical of U.S. labs. Replace them with the ranges on your report. Free T4 assays in particular differ between manufacturers, so a result near an edge can be flagged at one lab and not at another.
Some groups have their own ranges:
- Pregnancy. TSH falls in the first trimester. The 2026 ATA pregnancy guideline prefers trimester-specific ranges from the lab, and free T4 assays are less reliable in pregnancy.
- Older adults. TSH tends to rise with age in people without thyroid disease, so a mildly high TSH in an 80-year-old may be normal for them.
- People on levothyroxine. Your clinician may aim for a narrower band than the lab range.
How to track your TSH over time covers these in more detail, with sources.
What can throw the results off
- Biotin. High-dose biotin, common in hair and nail supplements, interferes with many thyroid immunoassays. It can make TSH look low and free T4 look high, mimicking an overactive thyroid. Tell the lab and your clinician if you take it.
- Timing after a dose change. TSH takes 4 to 6 weeks to settle after levothyroxine is started or changed. A result drawn sooner is still catching up.
- Recent illness. Serious illness can lower TSH and free T4 for weeks, and TSH can overshoot above range on the way back.
- Antibodies against the assay. Rarely, antibodies in a person’s blood react with the test itself and give a falsely high result. A lab can check by repeating the test on another platform.
Free T4 units
Free T4 is reported in ng/dL in the U.S. and pmol/L in most other countries. 1 ng/dL is 12.87 pmol/L. The tool converts the result and the range together when you switch units. TSH is the same number in mIU/L and µIU/mL. The lab unit converter has free T3, total T4, and total T3.
TSH and free T4 in HealthViewer
HealthViewer is a lab results tracker that runs on your own computer. It imports thyroid panels from lab report PDFs, MyChart downloads, and direct connections to your health system. In Trends, the overlay view puts TSH and free T4 on one chart, each on its own axis, so you can see whether they move in opposite directions. If your clinician gave you a target, set it as a custom range in the Test Catalog and the chart shades it instead of the lab’s range.
Everything stays in an encrypted file on your computer, with no account and no cloud service. Getting Started walks through the first import.
This tool describes patterns in lab results. It doesn’t diagnose thyroid disease. Talk to your clinician about your results and before changing any thyroid medication.