As a result, T4 is a much better marker of thyroid function.įree T4 (fT4) is roughly 1% of the total T4, with the rest being bound to thyroid-binding globulin. This is because the thyroid releases T4 and T3 at a ratio of about 20:1 respectively, with T3 mainly being produced by peripheral conversion of T4. Whilst free T3 (fT3) is measured, it is less relevant than free T4 (ft4). Reference ranges for TFTs often vary between labs, so always refer to your local guidelines. There are separate reference ranges for children and pregnant women. ![]() The term ‘ thyroid function tests‘ refers to the following investigations: Thyroid hormonal axis diagram Thyroid function tests (TFTs) This is the ‘brake system’ which aims to maintain a state of homeostasis. ![]() Thyroid hormonal axis overviewġ. The paraventricular nuclei in the hypothalamus release thyroid-releasing hormone (TRH).Ģ. This causes thyrotrope cells in the anterior pituitary to release thyroid-stimulating hormone (TSH).ģ. The thyroid responds to the TSH by releasing T4 and T3.Ĥ. T4 inhibits the pituitary and hypothalamus in a negative feedback loop. Each of the thyroid function tests assesses a different aspect of this axis and understanding the relationship between the results is key to being able to reach a diagnosis. The ability to interpret thyroid function tests (TFTs) first requires an understanding of the thyroid hormonal axis. You might also be interested in our medical flashcard collection which contains over 1000 flashcards that cover key medical topics.
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