T3

What is being tested?
This test measures the amount of triiodothyronine, or T3, in the blood. T3 is one of the two main hormones produced by the thyroid (the other is thyroxine, or T4). T3 makes up less than 10% of what is called “thyroid hormone”, while T4 makes up the rest. T3, however, is about four times more active than T4 and is thought to be responsible for most, if not all, of the effects of thyroid hormones.

Many cells in the body can convert T4 into T3; T4 can therefore be considered a hormone “reserve” that makes T3 available as the body needs it.
Thyroid hormones regulate the body’s metabolism (and other functions).

About 99.7% of T3 circulates in the blood bound to proteins, while the rest of the hormone is in free form. Both the free (unbound) form of the hormone and the total (bound plus free) form can be measured in a blood sample. However, measuring total hormone is considered obsolete, and measuring only the free fraction (free triiodothyronine, FT3) is recommended.

How is the sample collected for testing?
The blood sample is taken from a vein.

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
FT3 measurement helps define thyroid function and is carried out mainly to confirm a diagnosis of hyperthyroidism, as FT3 rises before FT4 and returns to normal later.
FT3 is usually of no use in hypothyroidism.

When is it requested?
Free T3 may be ordered when TSH and/or FT4 values are abnormal.

What does the test result mean?
There is no single reference range for this test. Reference values may differ between laboratories, as they depend on many factors, including the patient’s age and sex, the reference population and the analytical methods. Your laboratory report should include specific reference ranges for each test.

A high free T3 value may indicate an overactive gland (hyperthyroidism).
A low free T3 value may indicate an underactive gland (hypothyroidism).

FT3	FT4	TSH	Interpretation
Normal	Normal	High	Mild (subclinical) hypothyroidism
Low or normal	Low	High	Hyperthyroidism
Normal	Normal	Low	Mild (subclinical) hypothyroidism
High or normal	High or normal	Low	Hyperthyroidism
Low or normal	Low or normal	Low	Secondary (pituitary) hypothyroidism

 

If a patient with hyperthyroidism is being treated and has normal FT3 values (or, more often, normal FT4 or TSH), the treatment is probably working. If, however, FT3 (or FT4) values are still high, the treatment is not enough to bring the hormones back to normal levels, and symptoms of hyperthyroidism may persist.

Is there anything else I should know?
Many types of medicine – including oestrogens, certain types of contraceptive and high doses of aspirin – can affect total T3 test results. It is therefore advisable to tell your doctor about any medicines you are taking before the test. In general, free T3 levels are much less affected by these types of medicine.
During illness, the conversion of T4 to T3 decreases. Many people admitted to hospital have low T3 or free T3 levels. For this reason, T3 is not a routine test for hospital patients.

1. How can pregnancy change thyroid hormone levels?

1. How can pregnancy change thyroid hormone levels?
Pregnancy can raise total T3 (and total T4) levels. Free T3 levels do not normally change significantly. Even if total T3 levels are high, this does not mean that thyroid disease is developing.