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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 standard definition of a reference range for a particular measurement is defined as the interval between which 95% of values of a reference population fall into, in such a way that 2.5% of the time a value will be less than the lower limit of this interval, and 2.5% of the time it will be larger than the upper limit of this interval, whatever the distribution of these values.
Reference ranges (reference intervals) for blood tests are sets of values used by a health professional to interpret a set of medical test results from blood samples. Reference ranges for blood tests are studied within the field of clinical chemistry (also known as "clinical biochemistry", "chemical pathology" or "pure blood chemistry"), the ...
Free T4, total T3 or both are elevated and serum TSH is below normal in hyperthyroidism. If the hyperthyroidism is mild, only serum T3 may be elevated and serum TSH can be low or may not be detected in the blood. [14] Free T4 levels may also be tested in patients who have convincing symptoms of hyper- and hypothyroidism, despite a normal TSH.
TSH levels are considered the most sensitive marker of thyroid dysfunction. [84] They are however not always accurate, particularly if the cause of hypothyroidism is thought to be related to insufficient thyrotropin releasing hormone (TRH) secretion, in which case it may be low or falsely normal. In such a case a TRH stimulation test, in which ...
Also in normal pregnancies, adequate levels of maternal thyroid hormone are vital in order to ensure thyroid hormone availability for the foetus and its developing brain. [89] Congenital hypothyroidism occurs in every 1 in 1600–3400 newborns with most being born asymptomatic and developing related symptoms weeks after birth.
[22] [79] [87] As a result, LT 4 monotherapy may not result in a "truly biochemically euthyroid state." [23] Patients may express a preference for "low normal or below normal TSH values" [87] and/or T 4 and T 3 monitoring. The monitoring of other biomarkers that reflect the action of thyroid hormone on tissues has also been proposed. [15] [97] [22]
The disorder results from an antibody, called thyroid-stimulating immunoglobulin (TSI), that has a similar effect to thyroid stimulating hormone (TSH). [1] These TSI antibodies cause the thyroid gland to produce excess thyroid hormones. [1] The diagnosis may be suspected based on symptoms and confirmed with blood tests and radioiodine uptake.