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Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. [1] TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy.
The therapeutic target range TSH level for patients on treatment ranges between 0.3 and 3.0 μIU/mL. [18] For hypothyroid patients on thyroxine, measurement of TSH alone is generally considered sufficient. An increase in TSH above the normal range indicates under-replacement or poor compliance with therapy.
In pregnancy, subclinical hypothyroidism is defined as a TSH between 2.5 and 10 mIU/L with a normal thyroxine level, while those with TSH above 10 mIU/L are considered to be overtly hypothyroid even if the thyroxine level is normal. [44]
TSH levels are determined by a classic negative feedback system in which high levels of T3 and T4 suppress the production of TSH, and low levels of T3 and T4 increase the production of TSH. TSH levels are thus often used by doctors as a screening test, where the first approach is to determine whether TSH is elevated, suppressed, or normal. [25 ...
It is recommended that TSH levels are maintained below 2.5 mU/L in the first trimester of pregnancy and below 3 mU/L in later pregnancy. [22] The recommended maintenance dose of thyroxine in pregnancy is about 2.0-2.4 μg/kg daily.
T 3 is the more metabolically active hormone produced from T 4.T 4 is deiodinated by three deiodinase enzymes to produce the more-active triiodothyronine: . Type I present in liver, kidney, thyroid, and (to a lesser extent) pituitary; it accounts for 80% of the deiodination of T 4.
TSH – A thyroid-stimulating hormone level should be obtained first. If it is suppressed, then the nodule is likely a hyperfunctioning (or "hot") nodule. These are rarely malignant. FNAC – fine needle aspiration cytology is the investigation of choice given a non-suppressed TSH. [26] [27] Imaging – Ultrasound and radioiodine scanning.
Thyroid function tests include a battery of blood tests, including the measurement of the thyroid hormones, as well as the measurement of thyroid stimulating hormone (TSH). [84] They may reveal hyperthyroidism (high T 3 and T 4 ), hypothyroidism (low T 3 , T 4 ), or subclinical hyperthyroidism (normal T 3 and T 4 with a low TSH).