Free T4 (Thyroxine)

What is T4?

Thyroxine is one of two major hormones produced by the thyroid gland. T4 (thyroxine) is a hormone that is released in our blood that stimulates our metabolism, energy levels and overall well-being. Both thyroxine (T4) and triiodothyronine (T3) are created and stored in the thyroid. Thyroid hormones help regulate the body’s metabolism - that is, how the body functions.

Why do we analyse free T4?

Within the blood, most thyroid hormones are attached to a protein, but it is the hormones that are free from these proteins that are able to affect body functions. This is therefore why we measure the 'free' thyroxine in your blood sample. Analysing free T4 should be seen as a complement to testing TSH, allowing us to assess the degree of hypothyroidism or hyperthyroidism.

This is because a free T4 test is quite a non-sensitive measure of how your thyroid is functioning.

Meanwhile analysing TSH is a very sensitive measure to check the occurrence of free thyroid hormone. This is because a twofold change in free thyroid hormone is equivalent to a hundredfold change in TSH concentration.

Testing the TSH and free T4 combination is also needed to detect cases of pituitary/hypothalamus insufficiency. This is where the concentration of TSH is low, normal or slightly high AND the level free T4 is low.

A free T4 test is also recommended for therapy evaluation with thyreostatic drugs, as low TSH levels are to be expected for months or years after treatment commences.

Please note: Levels may differ during pregnancy.

High levels of free T4

High levels are seen in cases of hyperthyroidism (overactive thyroid).

Levels at the upper reference limit are usually seen when someone suffering from hypothyroidism (underactive thyroid) is being treated adequately.

Low levels of free T4

Low levels are seen in cases of hypothyroidism (underactive thyroid). In rare cases, patients may have low levels due to circulating antibodies against thyroxine.

Low levels can also be seen from the last trimester of pregnancy without hypothyroidism being present.


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