Investigating Thyroid Function
Range of tests available
TSH (thyroid stimulating hormone, thyrotropin) – In most situations TSH analysed using a high sensitivity assay is now accepted as the first line test for assessment of thyroid function. A TSH between 0.4 and 4.0 mIU/L gives 99% exclusion of hypo- or hyperthyroidism,12 while the TSH is considered more sensitive than FT4 to alterations of thyroid status in patients with primary thyroid disease.
FT4 (free thyroxine) – This test measures the metabolically active, unbound portion of T4. Measurement of FT4 eliminates the majority of protein binding errors associated with measurement of the outdated total T4, in particular the effects of oestrogen.
FT3 (free triiodothyronine) – FT3 has little specificity or sensitivity for diagnosing hypothyroidism and adds little diagnostic information. The main value of FT3 is in the evaluation of the 2 to 5% of patients who are clinically hyperthyroid, but have normal FT4. In this situation, an elevated FT3 would be suggestive of T3 toxicosis, in which the thyroid secretes increased amount of T3 or there is excessive conversion of T4 to T3.
Thyroid autoantibodies – The key reason for the measurement of these antibodies is almost entirely for the management of those with abnormal thyroid function. Autoimmune thyroid disease is detected most easily by measuring circulating antibodies against thyroid peroxidase and thyroglobulin (Thyroid peroxidase antibodies are also known as anti-TPO or antimicrosomal antibodies). In subclinical disease, the presence of thyroid antibodies increases the long-term risk of progression to clinically significant thyroid disease about two-fold. Almost all patients with autoimmune hypothyroidism and up to 80% of those with Graves disease have TPO antibodies, usually at high levels, although about 5 to 15% of euthyroid women and up to 2% of euthyroid men will also have thyroid antibodies.
Thyroglobulin – Levels are increased in all types of thyrotoxicosis, except thyrotoxicosis factita caused by self-administration of thyroid hormone. The main role for thyroglobulin is in the follow-up of thyroid cancer patients. After total thyroidectomy and radioablation, thyroglobulin levels should be undetectable; measurable levels (>1 to 2ug/L) suggest incomplete ablation or recurrent cancer.
Thyroid stimulating antibody - (Previously called long-acting thyroid stimulating antibodies or LATS) has a role in the diagnosis of Graves disease where other test results are ambiguous. It may also be useful in pregnant women with Graves disease, to determine the likelihood of fetal thyrotoxicosis.
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