Calculator inputs
Use the preset interval that best matches your situation, or switch to a custom interval from your own report.
Example data table
These examples show how the same lab value can be reviewed with different reference presets and converted units.
| Case | FT4 | Unit | Preset | Reference Interval | Status | Optional TSH |
|---|---|---|---|---|---|---|
| Adult A | 15.4 | pmol/L | Adult | 12.0–22.0 pmol/L | Within range | 2.10 mIU/L |
| Adult B | 0.72 | ng/dL | Adult | 0.93–1.71 ng/dL | Low | 6.80 mIU/L |
| Adult C | 1.92 | ng/dL | Adult | 0.93–1.71 ng/dL | High | 0.06 mIU/L |
| Pregnancy D | 13.2 | pmol/L | Pregnancy 2nd trimester | 10.0–17.0 pmol/L | Within range | 1.40 mIU/L |
| Custom E | 16.0 | pg/mL | Custom interval | 9.8–17.6 pg/mL | Within range | Not entered |
Formula used
This calculator does not create a laboratory free T4 result from scratch. Direct free T4 is usually measured by the lab. The calculator standardizes units, compares the value against a selected interval, and adds simple trend math.
- Unit conversion: pmol/L = ng/dL × 12.87
- Reverse conversion: ng/dL = pmol/L ÷ 12.87
- Range position: ((FT4 − low) ÷ (high − low)) × 100
- Midpoint deviation: FT4 − ((low + high) ÷ 2)
- Trend percent: ((current − previous) ÷ previous) × 100
How to use this calculator
- Enter your free T4 value exactly as shown on your laboratory report.
- Select the same unit used by your report before reviewing the result.
- Choose an adult or pregnancy preset, or type your own range.
- Add an earlier free T4 value if you want a trend check.
- Optionally enter TSH and its reference range for pattern context.
- Press the calculate button to show the result above the form.
- Download the summary as CSV or PDF for discussion or records.
Frequently asked questions
1) What does free T4 measure?
Free T4 measures the unbound portion of thyroxine in blood. That unbound fraction is available to tissues and is commonly reviewed with TSH when thyroid function is assessed.
2) Why does the calculator use reference ranges?
A number alone is hard to interpret without context. Reference intervals show whether the result falls below, within, or above the selected laboratory-style range.
3) Are the default ranges always correct for me?
No. Free T4 ranges vary by laboratory method, age, pregnancy status, and local validation. Use your own report range whenever it differs from the preset values.
4) Why can normal FT4 still appear with abnormal TSH?
TSH often shifts before FT4 leaves the range. That can happen in early or milder thyroid dysfunction, medication adjustment, or recovery from illness.
5) Does this tool diagnose hypothyroidism or hyperthyroidism?
No. It organizes values and patterns for review. Diagnosis depends on history, symptoms, medications, pregnancy status, antibody testing, repeat labs, and clinician judgment.
6) Can I compare older and newer free T4 values?
Yes. Enter a previous result and its unit. The calculator converts both values to one basis, then reports absolute and percentage change.
7) What if my unit is ng/dL instead of pmol/L?
Select the matching unit from the dropdown. The calculator converts values automatically and also shows the result in several common free T4 units.
8) When should I seek medical advice?
Contact a clinician when results are outside range, symptoms are significant, pregnancy is involved, medication changes are planned, or the pattern seems inconsistent.