Thyroid Blood Tests - Reference Ranges are Not 'Normal Ranges'
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.
Just because a person's thyroid hormones happen to fall somewhere within the wide population range does not mean they are adequately treated.
The reference range is 'the broad side of a barn' and whether it is 10 feet wider or narrower, it is still the broad side of a barn. Thyroid function tests all suffer from a ‘low index of individuality’. The range of variance in the population is much wider than the range of variance in the individual (this is also known as non-ergodicity). We know now from research that the actual individual person ranges for levels like FT3 and FT4 are less than half as wide as the wide population ranges that doctors are using to assess our thyroid lab results. When you make generalisations based on the population and try to apply them to the individual, you have problems.
If I use the 'barn door' analogy again: imagine that a thyroid patient could draw his/her own small circle in chalk on the barn door. The circle represents the individual range for where their individual FT3 needs to be for them to be healthy. It might have been lower on the barn door before they developed thyroid disease, but imagine it is in the right place now. Only if their FT3 hits within the circle will they feel well. The lab reference range (the barn door) doesn't help much in assessing their result. They can throw a ball at all places on the barn door, but only if they hit it within their own circle will it the FT3 make them feel well. We also don't know from any lab test result where their individual circle actually is!
Thyroid blood tests can be useful only if you understand their limitations. The primary fallacy is calling the reference range the ‘normal range’. This leads to doctors saying, "you’re results are normal, therefore it can’t be a thyroid problem". A false assumption leads to an inaccurate diagnosis.
The reference ranges have never been capable of ‘pinpoint accuracy’ - they are just probability curves that never reach 100%. They are not and never have been ‘certainty’ curves.
This laboratory-test approach that modern medicine uses is more suited to a fast, conveyor-belt, production-line processing of patients, than it is to providing each, individual patient with advice and help that is uniquely tailored to them.
In addition to all the above the thyroid lab tests cannot assess what is occurring within the cells - where significant amounts of conversion of T4 to T3, and binding to thyroid receptors needs to occur.
Where thyroid results fall within reference ranges can only be a guide. Symptoms and signs are paramount, and how the patient's symptoms alter under treatment and how this relates to changes in thyroid lab test results is important. The biggest mistake that is made with lab test results and reference ranges is to assume that an individual is properly treated simply because their results fall within these ranges. This mistake often leave patients under-medicated and sometimes on the wrong type of thyroid medication.
This entire situation is like you pushing your car into a garage because it is coughing and spluttering and not working right. If the car mechanic turned the ignition on and told you, "Everything is fine - look you have enough fuel in the tank on the gauge!", you would tell him he was nuts and that you had an engine issue of some kind. That's all the thyroid blood tests can do. They are a gauge of what's in the blood and because of that, they are not the perfect viewing tool on thyroid hormone action within the cells that doctors would have you believe.
Do not believe everything you are told about your thyroid hormone levels based on your thyroid lab results!
(Updated in February 2019)