Thyroid Blood Tests - Test Results for Patients on T3-Only

In a thyroid hormone blood test, the levels of TSH, FT4 and FT3 in the bloodstream are measured from the blood sample. It is impossible for a thyroid blood test to reveal the actual levels of FT4 and FT3 within the cells, and whether cell function is actually being regulated correctly. A blood sample cannot reveal any of these things, no matter how much we may want it to. Thyroid blood tests can only, at best, provide an estimate of might be happening with thyroid hormones within the tissues. This approach is helpful for people on T4, T4/T3 or even natural desiccated thyroid, as long as some of the limitations and mathematics of the population reference ranges are remembered.

These thyroid blood tests are quite useful for the initial diagnosis of a thyroid problem. However, during treatment with T3-Only, thyroid blood tests provide no useful insight into actual cell regulation.

What do I mean by 'cell regulation'? I use this term a great deal in the book 'Recovering with T3'.

Each cell has a cell nucleus and a mitochondrion. The cell nucleus amongst other things produces the proteins that are specific to that type of cell. The cell nucleus requires the right amount of T3 to bind with its thyroid receptors for it to operate at the right rate for good health. The nucleus also needs the right amount of chemical energy (ATP) available and a range of other nutrients and resources, including cortisol The ATP must be produced by the mitochondrion in each cell and this requires T3, glucose and a range of nutrients. 

When cells are regulated properly by thyroid hormone they will be running optimally and our overall health will be good. A body with properly regulated cells will have good energy, no weight issues, healthy blood pressure, good skin, hair and nail condition, a normal body temperature, a healthy digestive system, i.e. they will have no overt hypothyroid or hyperthyroid symptoms.

Some people argue that if TSH falls within the reference range, the tissues are getting an adequate supply of thyroid hormones. However, this assumption has been shown to be incorrect by medical research over the last decade.

The pituitary gland has no insight into what is happening deep within the other cells of the body. It can only measure bloodstream levels of FT4 and FT3, and can only make sense of these as long as they are in the types of ratios that it has evolved to assess. The pituitary also converts its own T4 to T3, and has the highest concentration of T3 out of all the body tissues. The pituitary gland is simply assessing the contents of the bloodstream as it flows through it. It has no insight into the rest of our tissues.

FT4 and FT3 in the bloodstream cannot be assumed to be representative of the correct level of FT3/FT4 required to regulate cell function correctly. There are many technical reasons for this and this is discussed at length in 'Recovering with T3'. However, many thyroid patients have FT4 and FT3 levels that fall within the population ranges,, and find that they are still ill, and only regain their health on T3-Only, with quite unusual FT4 and FT3 levels. I fall completely into this category, and so do many others.

Thyroid blood tests, no matter how ‘normal’ they may appear, cannot reveal whether the cells are actually getting the levels of thyroid hormones that they need. Thyroid blood tests are unable to indicate how well thyroid hormones are entering the cells. Thyroid blood tests cannot assess how efficient the conversion of FT4 to FT3 is within the cells. Thyroid blood tests cannot assess how much of the biologically active T3 hormone eventually reaches those critical parts of the cells where it is needed. 

To make matters worse, for those on T3-Only treatment, FT3 fluctuates so significantly in the bloodstream that the timing of a thyroid blood test in relation to when the last T3 dose was taken can have a profound effect on any FT3 and TSH result. If the timing of the blood draw relative to the last T3 dose is too close, it can leave FT3 well over the top of the reference range and TSH totally suppressed. If the gap between the last T3 dose and the blood draw is longer, FT3 can be within range and TSH not suppressed. The results are usually not meaningful in terms of whether the T3 dosage is correct or not.

What does all of this mean?

Thyroid blood tests are critical during the diagnostic stage of thyroid problems. For most thyroid patients on T4 or T4/T3 treatment, thyroid blood tests provide some insight into thyroid hormones that may be used as a  guideline during treatment. The only caveat is that the patient's doctor needs to be aware of the limitations of the laboratory tests and how to use them. Symptoms and signs still need to be the most important indicator of whether the dose and type of thyroid replacement is the right one for the patient.

However, for those patients that have responded badly to T4 and T4/T3, and need T3-Only to regain their health, they are unlikely to be served well by using thyroid blood tests to manage T3 dosage levels. 

Some doctors still believe that they can use TSH, FT3 and FT4 to manage T3-Only thyroid treatment. Some doctors even insist on having the levels of these hormones in the bloodstream fit within the standard reference ranges. This is absurd! Trying to apply reference ranges that have been developed for healthy people to those that are T3-Only no sense at all. There is a particular issue with rigidly applying the FT3 reference range and attempting to manage a person's T3 dosage so that FT3 always remains within it. But I will return to this in a later blog post, as it is a complex topic to explain.

Thyroid patients on T3-Only who have their T3 medication managed via thyroid laboratory tests and reference ranges, which have been designed for normal healthy people who can process T4, may be condemned to permanent ill health.

Other methods need to be employed until a time comes when thyroid researchers have provided more useful tests that reflect how well cells are actually being regulated by T3.

All of the above is explained in more detail in 'Recovering with T3'.

Best wishes,


(Updated in January 2019)