Free T4 and free T3: reading the active thyroid hormones

If TSH is the order sent to the thyroid, free T4 and free T3 are the delivery. They are the hormones that actually act on your cells and set how fast your body runs. Measuring them alongside TSH turns a single clue into a clear picture.

T4 and T3: two forms of the same hormone

The thyroid mainly makes T4, also called thyroxine. T4 is fairly inactive on its own. It acts as a reservoir that the body converts into T3, the more powerful form, inside tissues such as the liver, kidneys and muscles. Most of the T3 in your blood comes from this conversion rather than straight from the thyroid.

The numbers refer to how many iodine atoms each hormone carries: four for T4 and three for T3. This is why the thyroid needs iodine from the diet, and why iodised salt matters.

Why “free” matters

More than 99% of thyroid hormone in the blood is bound to carrier proteins. Only the tiny unbound, or free, portion can enter cells and do its job. Free T4 and free T3 tests measure just this active part.

This is important because the carrier proteins change with pregnancy, the contraceptive pill, liver disease and some medicines. Total hormone levels move with the proteins, while the free levels give a truer reading of what your body can use.

The units and ranges

South African labs report free T4 and free T3 in pmol/L. The ranges vary by lab and method, so LifePrint uses the range printed on your lab report. Comparing results over time is easiest when you use the same lab.

Reading the patterns

The real value comes from reading TSH, free T4 and free T3 together. Some common patterns:

• High TSH with low free T4: an underactive thyroid, known as hypothyroidism.
• High TSH with normal free T4: subclinical hypothyroidism, often an early or mild change that is repeated before any decision is made.
• Low TSH with high free T4, free T3 or both: an overactive thyroid, known as hyperthyroidism.
• Low TSH with normal free T4 and free T3: subclinical hyperthyroidism.
• Low or normal TSH with low free T4: uncommon, and can point to a problem with the pituitary gland rather than the thyroid. This needs a doctor's assessment.

Why free T3 is measured

Free T3 is most useful when an overactive thyroid is suspected. In some people, especially early on, the thyroid makes too much T3 while T4 stays normal. Measuring free T3 catches these cases, which free T4 alone would miss.

In an underactive thyroid, free T3 is often the last value to fall, because the body works hard to keep converting T4 into T3. A normal free T3 therefore does not rule out an underactive thyroid.

Things that can affect the results

• Biotin. High-dose biotin can make free T4 and free T3 read falsely high and TSH falsely low, which can look like an overactive thyroid. Pause it for 48 hours before your test.
• Thyroid medication. Levothyroxine taken shortly before a blood test can raise free T4 for a few hours. Ask your doctor whether to take your dose before or after the draw on test day.
• Other medicines, including amiodarone, heparin and high-dose steroids.
• Pregnancy, which changes both the hormone levels and the ranges used to judge them.

What happens next

Your LifePrint report reads your free T4 and free T3 alongside TSH and TPO antibodies, explains the pattern in plain language and flags how urgently to follow up. If your results suggest the thyroid is under or overactive, discuss them with a registered healthcare practitioner, who can confirm the finding, look for a cause and advise on any treatment.