![]() If TSH is normal, abnormal thyroid FT4 is unlikely based on the committee’s experience. ![]() If TSH is below the reference range both FT4 and FT3 will be necessary as the results of both FT4 and FT3 will dictate decisions around which type of treatment the thyrotoxicosis may require. In the first investigation of possible thyroid disease the committee agreed based on their experience that a testing strategy of only using TSH with a FT4 measurement if TSH is above the reference range would be appropriate for the majority of adults. The purpose of this review is to establish if objective evidence is available to support the premise that the use of more complex thyroid function testing strategies leads to demonstrable health benefit. Given the widespread use of TFTs the addition of FT4 and FT3 to the TFT represents a considerable financial burden to the UK health economy. The authors recognised the need for further studies in particular the use of TFTs as a screening test and for diagnosing and treating subclinical disease. In other situations, such as longer term follow-up, the testing of TSH alone may be appropriate. This guide recommends using both TSH and FT4 at the first investigation of thyroid disease, at the initial optimisation of therapy in both hyper or hypothyroidism, in patients treated with thionamides, in pregnancy or when pituitary disease is suspected. The latest authoritative UK guidance on TFT came from the British Thyroid Association in 2006. Other indications for FT3 include exclusion of T3 toxicosis (when TSH is suppressed but FT4 is within the reference interval), the detection of non-thyroidal illness, to adjust levothyroxine dosage if TSH remains elevated in the presence of FT4 concentrations above the reference interval or more contentiously to direct thyroid replacement therapy in patients with TSH within the reference interval but with persistence of hypothyroid symptoms. Of the laboratories that offer FT3 analysis, some provide open access where others will use FT3 instead of FT4 to confirm the diagnosis of hyperthyroidism when TSH is suppressed. Many laboratories offer FT3 testing but this is not universally, available. Common protocols include an initial TSH measurement with thyroid hormone analysis (either free thyroxine - FT4 or free triiodothyronine - FT3) cascaded when TSH is outside the established reference interval, simultaneous analysis of both TSH and FT4 or open access to either test depending on requestor preference. Various TFT protocols are used in current clinical practice with no particular procedure dominating. This avoids situations where variation in thyroid hormone binding proteins rather than thyroid function per se is responsible for abnormal test results. Most UK labs would advocate the measurement of free thyroid hormones rather than total thyroid hormone (thyroid hormone that is not bound to thyroid carrier proteins) as the best marker for biologically active hormone. For these reasons the direct analysis of thyroid hormones is also recommended as an adjunct to TSH testing. ![]() Both pituitary deficiency and autonomous TSH production by pituitary adenomas, can lead to erroneous classification of thyroid function if serum TSH is used as a sole biomarker. Secondary thyroid dysfunction due to pituitary disease is a less common confounder of TFTs. This is most frequently encountered when the homeostatic mechanism has been impaired by long standing primary thyroid disease or the pituitary-thyroid axis has not reached equilibrium following changes to thyroid therapy. However there are notable exceptions when serum TSH concentration alone may not accurately reflect thyroid hormone production. TSH is secreted by the pituitary gland in response to circulating thyroid hormone concentration in a classic endocrine feedback loop and can therefore be used as a marker of thyroid status. Measurement of serum TSH concentration is considered as the most effective single marker for the exclusion of primary thyroid dysfunction. ![]() Despite their widespread use and high clinical efficacy, there is still considerable debate as to the optimum testing strategy for both the diagnosis and monitoring of thyroid dysfunction. Consequently TFTs are amongst the most widely requested blood tests and are the first line investigation when thyroid disease is suspected. These biochemical tests have both high analytical sensitivity and specificity and well established clinical utility. TFTs are used for diagnosis and to monitor treatment of common thyroid gland disorders. A Thyroid function test (TFT) commonly refers to the quantitation of thyroid stimulating hormone (TSH) and circulating thyroid hormones in serum to assess the ability of the thyroid gland to produce and regulate thyroid hormone production.
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