Your thyroid makes hormones that are important for many of your body's functions. How your thyroid is working affects your metabolism, heart rate, blood pressure and body temperature.   Metabolism refers to all the physical and chemical processes in the body that convert or use energy for breathing, blood circulation, body temperature control, brain and nerve function and more.

There are two main hormones produced by the thyroid. These are T4 and T3.  They circulate in your blood and it is important that levels stay constant to keep your metabolism running and in balance.  There is a feedback loop to make sure they don't get too high or too low.

• _{Your thyroid is under the control of your hypothalamus, which is part of your brain, and your pituitary gland which sits at the base of your brain.}
• _{The pituitary gland tells your thyroid gland how much thyroid hormone to make and release and it does this by controlling the blood levels of TSH (Thyroid Stimulating Hormone).}   
• _{If thyroid hormone production falls, TSH rises and conversely,} 
• _{If thyroid hormones become too high, TSH levels fall.}

If you have symptoms that suggest you have a thyroid problem your doctor will start by ordering a Thyroid Stimulating Hormone (TSH) test. 

If your TSH level is high or low, you may need to have a Free T4 test to identify the problem and sometimes a free triiodothyronine (FT3) test may also be requested. 

T3 makes up less than 10 percent of thyroid hormones produced but it is about four times as strong as T4.

A ‘free’ T4 or T3 test refers to the fact that hormones are circulating freely and available to be absorbed by body tissues.

Reference intervals

Your results will be compared to reference intervals (sometimes called a normal range).

• Reference intervals are the range of results expected in healthy people 
• When compared against them your results may be flagged high or low if they sit outside this range.
• Many reference intervals vary between labs so only those that are standardised or harmonised across most laboratories are given on this website.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation. Your results need to be interpreted by your doctor.