A laboratory reference range is the range in the unit of measurement for the given test that is usually provided with a specific test result. It is usually expressed as lowest level - highest level.
Keeping a record of all test results including the reference ranges can be very useful. However, references ranges are frequently misused.
Laboratories assess a wide population of people when calculating the reference ranges for TSH, FT4 and FT3. This is why they are population ranges. They do not provide the range that is necessarily where an individual's results need to be.