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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. 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.
Frequency. 0.3–0.4% (USA) [8] Hypothyroidism (also called underactive thyroid, low thyroid or hypothyreosis) is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormones. [3] It can cause a number of symptoms, such as poor ability to tolerate cold, extreme fatigue, muscle aches, constipation, slow ...
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.
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). 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).
The initial diagnostic evaluation will start with plasma thyroid-stimulating hormone (TSH) concentration. If elevated, it signifies hypothyroidism. The elevation is usually a marked increase over the normal range and is generally greater than 20 mg/dl. Free T4 levels will usually be lowered, but sometimes might be normal.
The test is used in the differential diagnosis of secondary and tertiary hypothyroidism. First, blood is drawn and a baseline TSH level is measured. Then, TRH is administered via a vein. After 30 minutes blood is drawn again and the levels of TSH are measured and compared to the baseline. Some authors recommend additional blood sampling at 15 ...
Affected patients may have normal, low, or slightly elevated TSH depending on the spectrum and phase of illness. Total T4 and T3 levels may be altered by binding protein abnormalities, and medications. Reverse T3 levels are generally increased, while FT3 is decreased. FT4 levels may have a transient increase, before becoming subnormal during ...
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