What is the Difference Between T3 T4 TSH and FT3 FT4 TSH?

T3, T4, TSH, FT3, FT4, and TSH are all related to thyroid function and can be measured in blood tests. The main differences between these tests lie in the types of thyroid hormones they measure and their binding status to proteins in the blood. Here's a breakdown of each test:

• T3, T4, and TSH: These tests measure the levels of the thyroid hormones triiodothyronine (T3) and thyroxine (T4), as well as thyroid-stimulating hormone (TSH). T3 and T4 are bound to thyroxine-binding proteins in the blood, while TSH is a pituitary hormone that regulates the release of hormones from the thyroid gland.
• FT3, FT4, and TSH: These tests measure the levels of unbound or "free" thyroid hormones (FT3 and FT4) and TSH. Only unbound thyroid hormones are active and have an effect on the body. FT3 stands for "free triiodothyronine," and FT4 stands for "free thyroxine".

Both T3 T4 TSH and FT3 FT4 TSH tests are used to assess thyroid function and diagnose potential thyroid diseases. They are often used together to provide a more comprehensive picture of thyroid health. Some common conditions that can be diagnosed using these tests include hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid).

Comparative Table: T3 T4 TSH vs FT3 FT4 TSH

Here is a table comparing T3, T4, TSH, FT3, FT4, and TSH:

T3 and T4 are thyroid hormones, while TSH (thyroid-stimulating hormone) is a hormone that stimulates the thyroid gland to produce T3 and T4. FT3 and FT4 are the free, unbound forms of T3 and T4, respectively, which are biologically active and can enter cells to exert their effects.

Both T3/T4/TSH and FT3/FT4/TSH are types of thyroid tests used to diagnose and monitor thyroid disorders, such as hyperthyroidism and hypothyroidism. The tests have different sensitivities and specificities for certain conditions, and they may be used in combination to provide a more comprehensive assessment of thyroid function. Some common scenarios where these tests are used include:

• Investigating possible thyroid disorders (hyperthyroidism and hypothyroidism)
• Monitoring the adequacy of thyroid hormone replacement therapy
• Monitoring anti-thyroid treatment
• Investigating the hypothalamic-pituitary-thyroid axis

It is essential to note that FT3 and FT4 tests may be misleading in patients with non-thyroidal illnesses and should be delayed for at least three months post-acute illness. On the other hand, TSH tests are not reliable markers of thyroid status in the presence of suspected or confirmed central hypothalamic-pituitary dysfunction, and FT4 (±FT3) levels must be used instead.