Thyroid hormone is needed for energy production by every cell of the body. If the cells are not getting enough hormone, then nothing in the body works properly.  The same effect is suffered by all other glands, including the adrenals (adrenal glands), which secrete an important hormone called cortisol. Although thyroid hormone is referred to in the singular, sometimes referred to as just thyroid, there are two main hormones secreted by the thyroid, T4 and T3, as well as small amounts of T2, T1, T0 and Calcitonin. The thyroid mostly secretes T4, which functions as a storage hormone and is therefore inactive, and a much smaller amount of T3, which is the active hormone. These circulate in the bloodstream bound to carriers. The very small amounts of each hormone, free and ready for use in the body, are: free T4 and free T3. This is what is measured in the thyroid function test.

Both free T4 and T3 in the blood enter the cells. T3 goes straight to the nucleus of the cell receptors and binds there, a bit like a key fitting in a lock. Once this key is turned all sorts of reactions begin. T4 is changed or converted into T3 by the deiodinase enzymes. Some of this T3 also goes to the cell receptors to be used by the cells, the remainder goes out of the cell into the circulation. This conversion process may not work very well, even with the person’s own natural T4. In people treated with levothyroxine, a synthetic form of T4, the level of T3 tends to be lower than it would be naturally. Therefore, synthetic T3 is also given, although often not enough, and blood test results of people on combination therapy are never going to look like those of healthy people.

TSH is the signal sent by the brain to the thyroid to produce more hormone. If the brain thinks the level of hormone is too low, the TSH rises, as in hypOthyroidism. If however, the brain thinks that the level of hormone is too high, then the TSH goes right down, as in hypERthyroidism. In healthy people the TSH response varies from person to person, as the brain and the body work together. The response by the thyroid may become blunted as you become more hypothyroid. Please note: the low TSH of hypERthyroidism, the speeded up and less common form of illness, is not to be confused with the low TSH produced by adequate thyroid hormone replacement. Unfortunately, endocrinologists frequently confuse the two! Also, if your TSH is anywhere in a very wide reference range, that is deemed okay by most endocrinologists, as many of us have experienced. They are treating a test, not a patient. TSH does not correspond with symptoms. You can have a very high one but not be that unwell. It can be within the reference range, but yet a person can be very ill.

There is one more hormone that is sometimes measured (rarely by the NHS), and this is called reverse T3 or rT3. This is produced by one of the deiodinase enzymes, referred to above, from T4. It inactivates the nuclear receptor and acts as a kind of valve to prevent too much active T3 getting to the nucleus. People taking T4 orally, particularly levo, may make too much rT3, which shows up as poor conversion in a blood test. Poor conversion is when you have a low amount of T3 relative to T4 – this is why some endos give T3, although not enough usually. Excess rT3 can cause hormone resistance as it blocks the receptors at the nucleus. If you are very ill you may also produce a lot of rT3. Note that NDT also contains T4.