Triiodothyronine (T3) and Thyroxine (T4) are hormones produced by the thyroid and are responsible for regulating metabolism. Their expression is carefully controlled by thyroid-stimulating hormone (TSH) and they circulate in a biologically inactive form in the bloodstream to further restrict their activity. Thyroid dysfunction can lead to the production of too much to too little T3 or T4 causing hyper or hypothyroidism.

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Triiodothyronine (T3) And Thyroxine (T4)

The thyroid gland produces 3 hormones; calcitonin, thyroxine (T4), and triiodothyronine (T3). T3 and T4 have similar functions and are involved in the control of metabolism, whereas calcitonin is involved in regulating the levels of blood calcium.

T4 is the major form of the hormone and is converted into active T3 by deiodinases (5’-deiodinase). Although circulating levels of T4 are much greater than T3 levels, 99% of T4 remains bound and inactive. Overall, T3 possesses more biological activity than T4 (3-4 times more) and its effect is briefer due to its shorter half-life compared to T4.

Several diseases cause thyroid dysfunction in including hyperthyroidism, Graves’ disease, hypothyroidism, Hashimoto’s disease, thyroid nodules, and thyroid cancer. Individuals with Type 1 or Type 2 diabetes are at a higher risk of developing thyroid disease and it is believed that Type 2 diabetes can also induce thyroid disease as a result of chronically high blood sugar levels.


To screen for hyper and hypothyroidism, a T4 test and TSH test are commonly carried out to determine their serum levels. A total T4 test measures both the bound and free forms of the hormone and a free T4 measures the unbound, biologically active form.

Elevated levels of TSH and low free T4 indicate primary hypothyroidism due to a problem with the thyroid gland, and low levels of both TSH and free T4 also indicate hypothyroidism but due to a problem involving the pituitary gland.

In hyperthyroidism, low TSH levels with an elevated free T4 are typically found and a follow-up T3 test is used to confirm hyperthyroidism and determine its severity. Patients who are hyperthyroid will have an elevated T3 level. Total and free T3 and T4 serum levels are measured using competitive quantitative immunoassays.

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