Gp.unimelb.edu.au

Thyroid function tests
Robin H Mortimer, Professor, Department of Endocrinology, Royal Brisbane and Women's
Hospital, and the University of Queensland, Brisbane
About 90% of thyroid hormone released is T4 and 10% is T3. In some hyperthyroid states the ratio of T3 to T4 is higher. Both Thyroid disorders can be difficult to detect
hormones are co-secreted with thyroglobulin and circulate clinically, but thyroid function tests can assist
in blood bound to thyroid hormone binding proteins (thyroid in making a diagnosis. Measuring thyroid
binding globulin, transthyretin and albumin). A very small stimulating hormone is the first step. if it is
unbound ('free') fraction is available for uptake by cells. Much abnormal, free thyroxine should be measured.
of the T3 in the blood is generated by the liver after enzymatic removal of an iodine atom from T A raised concentration of thyroid stimulating
TSH secretion is mainly regulated by circulating T hormone with a low concentration of free
deiodinated to T3 in the pituitary) and to a lesser extent by thyroxine suggests hypothyroidism. A low
circulating T3. There is a classical negative feedback loop between concentration of thyroid stimulating hormone
T4 and TSH. This is log-linear (log TSH is inversely proportional with a high concentration of free thyroxine
to free T4), which means that small changes in free T4 cause large suggests hyperthyroidism. Measuring thyroid
autoantibodies may help establish the cause
Fig. 1
Pituitary-thyroid physiology
of the dysfunction. Different assays can give
different results, and tests of thyroid function
may be affected by drugs and intercurrent illness.
Key words: thyroxine, triiodothyronine, thyroid stimulating introduction
(T3). These hormones are essential for normal growth, Altered thyroid function is common. For example, the prevalence of hypothyroidism may be up to nearly 10% of the general population.1 As thyroid disorders may not present with The hypothalamic hormones thyrotrophin releasing classical clinical signs, it is essential to have accurate assays of hormone and somatostatin stimulate or block secretion of thyroid function to assist in the diagnosis.
thyroid stimulating hormone (TSH). TSH stimulates iodide Thyroid physiology (Fig. 1)
uptake by the thyroid and synthesis of the thyroid hormones thyroxine (T The thyroid gland actively transports diet-derived iodide from 4) and triiodothyronine (T3). T4 and T3 circulate bound to the thyroid hormone binding proteins (thyroxine the blood by means of a cell membrane iodide pump called the binding globulin, transthyretin and albumin). A very small sodium-iodide symporter. Iodide then combines with tyrosines free fraction of thyroid hormone is available for cellular in thyroglobulin, mediated by thyroperoxidase, to form T4 4 is deiodinated in liver and other tissues to form 3 (3 iodine atoms). The uptake of iodide and 4 and T3 are enhanced by thyroid stimulating pituitary to T3 which inhibits TSH secretion.
hormone (TSH) which is secreted by the pituitary gland.
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inverse changes in TSH concentrations. TSH secretion is also and mild hypothyroidism increases with age.
regulated by the hypothalamic hormones thyrotrophin releasing The concentration of thyroperoxidase antibodies may fluctuate hormone (stimulating) and somatostatin (inhibiting).
in patients with autoimmune thyroid disease. This has no clinical significance and repeated measurements are not Blood tests relevant to thyroid disease
recommended. Maternal thyroperoxidase antibodies cross the TSH is the hormone which is usually tested. It is the only test placenta, but their effects on fetal thyroid function are unclear.
funded by the Medicare Benefits Scheme to screen for thyroid disease when there is no history of thyroid problems.
Thyroglobulin autoantibodiesThyroglobulin autoantibodies are also a marker of autoimmune Thyroid stimulating hormone
thyroid disease, but are less common than thyroperoxidase TSH is a sensitive marker of thyroid function because it is antibodies. Thyroglobulin autoantibodies do not inhibit influenced by small changes in free T4 concentrations. A low thyroperoxidase or mediate antibody-dependent cell cytotoxicity TSH usually indicates hyperthyroidism whereas raised TSH and are therefore markers rather than mediators of autoimmune usually means hypothyroidism. over the years the lowest thyroid disease. There are considerable variations in sensitivity and concentration of TSH which can be detected by assays has reference ranges between assays. other autoimmune diseases can progressively fallen, allowing better separation of normal and also increase the concentration of thyroglobulin autoantibodies. Thyroid hormone assays
TSH receptor autoantibodies may stimulate or less commonly only very small fractions of thyroid hormones are not bound block the TSH receptor. Stimulating antibodies cause Graves' to protein. These free thyroid hormones are the physiologically disease and probably also cause the associated ophthalmopathy. important thyroid hormones in blood. Modern immunoassays Blocking antibodies can cause hypothyroidism. The assay of that estimate free hormone concentrations are widely available. TSH receptor autoantibodies done in clinical laboratories cannot Changes in serum albumin concentrations, abnormal binding distinguish between stimulating or blocking antibodies. This is proteins, free fatty acids and drugs such as heparin, frusemide not usually relevant as clinical hyperthyroidism would suggest and phenytoin may interfere with these assays. Most that the dominant antibody is stimulatory.
laboratories now use chemiluminescent methods that are more Measuring TSH receptor autoantibodies can be useful if the (but not completely) resistant to such interference. When results cause of hyperthyroidism is not apparent. However, initial do not fit into a recognised pattern the laboratory should be hopes that remission of Graves' could be predicted by falling consulted to identify such interferences.
autoantibody levels have not been supported by most studies. Measurements of TSH receptor autoantibodies do have an Thyroid-related autoantibodies
important role in managing pregnant women with Graves' If a person has altered thyroid function, testing for thyroid disease. High concentrations of maternal TSH receptor antibodies helps to determine if they have an autoimmune autoantibodies can predict fetal and neonatal hyperthyroidism. It is important to recognise that TSH receptor autoantibodies do not always fall after successful treatment, so pregnant women with a previous history of Graves' disease should be screened Thyroperoxidase antibodies are also known as thyroid microsomal antibodies. They are present in autoimmune thyroid disease, but there is debate about whether low levels are always Thyroglobulin
pathological. Unfortunately, there are significant differences Thyroglobulin, a large glycoprotein, represents about 80% of between laboratories when the same sera are studied, and the wet weight of the thyroid and is co-secreted with thyroid lower detection limits are variable. Assay sensitivities and hormone. Concentrations are high in patients with raised TSH reference ranges can therefore vary quite widely. concentrations or nodular goitres, but it is not clinically useful to Thyroperoxidase antibodies can cause hypothyroidism in at measure thyroglobulin in these situations.
least two ways. Firstly they can block thyroperoxidase thereby Most papillary and follicular carcinomas synthesise and secrete inhibiting T4 and T3 synthesis and secondly through antibody- thyroglobulin, but raised thyroglobulin levels are not a reliable dependent cell cytotoxicity and thyroid inflammation. Low indicator or screening test for thyroid malignancy. Thyroglobulin concentrations may not be associated with evidence of thyroid concentration becomes a useful marker of remaining or recurrent dysfunction, but the incidence of raised TSH increases as cancer in patients who have had a total thyroidectomy and remnant antibody levels rise. The prevalence of positive antibody levels ablation with radioiodine for papillary and follicular carcinoma. www.australianprescriber.com
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Unfortunately, up to 20% of patients with differentiated thyroid Detecting and confirming thyroid dysfunction
cancer have thyroglobulin autoantibodies that interfere with the thyroglobulin assay, leading to underestimation of thyroglobulin The inverse log-linear relationship between free T4 and TSH concentration. Thyroglobulin autoantibodies should therefore be means that TSH concentrations are sensitive indicators of measured, with a sensitive assay, on all thyroglobulin samples.
thyroid dysfunction. A raised TSH suggests hypothyroidism2 Reference ranges
while a low TSH suggests hyperthyroidism. There are other causes of low TSH concentrations, notably hypothalamic- As most commercial assays do not physically measure the analyte, pituitary disease, but this is very uncommon in the general results given are always an approximation of actual levels. Each population. The finding of an abnormal TSH should lead to assay, even for the same analyte, will therefore give slightly different results because of intrinsic variations in the reagents used Interpretation of thyroid function tests may be particularly and the effects of interfering illnesses and substances. Free T3 difficult if the patient is systemically ill. Starvation or severe levels are the most variable between assay methods.
illness can be associated with dysregulation of TSH secretion Reference ranges are altered by ethnicity, age and iodine intake. and reduced deiodination of T4 to T3 (the 'sick euthyroid' In Australia these factors are probably not clinically significant. syndrome). Low TSH and T3 levels are typical and can cause Different ranges also apply in pregnancy, neonates and very Very occasionally a raised TSH with a normal free T4 relates Reference ranges are defined as those into which 95% of a normal to interference in the TSH assay. Very rarely, thyroid hormone population fall. (Accordingly 2.5% of normals will have higher and resistance or a pituitary TSH-secreting adenoma is associated 2.5% will have lower results than the reference range.) Each assay with a mildly raised TSH in the presence of a raised free T4.
must therefore be interpreted in terms of its own reference range. Treatment with amiodarone is often associated with abnormal The practical implications of this are that blood test results from thyroid function tests. The most common finding is a raised different laboratories may not be directly comparable and their TSH caused by inhibition of pituitary T4 to T3 conversion, but interpretation requires examination of the reference ranges. true hypothyroidism and hyperthyroidism can occur. Diagnosis Reference ranges change in pregnancy. In early pregnancy and management may be complex and require expert advice.
chorionic gonadotrophin is secreted by the placenta in large amounts. This is structurally similar to TSH (but is not measured Hyperthyroidism
by the TSH assay) and stimulates the maternal thyroid. This leads A low TSH and raised free T4 indicate hyperthyroidism and to increased maternal thyroid hormone secretion and a reduced should lead to consideration of causation and treatment. The maternal TSH. occasionally women develop mild hyperthyroidism majority of younger patients will have Graves' disease, but older in the first trimester, especially if they have hyperemesis. patients are more likely to have nodular thyroid disease. Table 1
Common results of thyroid function tests
free tri-
Thyroperoxidase
stimulating
thyroxine
iodothyronine
and thyroglobulin
autoantibodies
Hyperthyroidism (consider Graves', measure TSH receptor autoantibodies) Subclinical hyperthyroidism (consider nodular thyroid disease) | VoLUMe 34 | NUMBer 1 | feBrUArY 2011
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Transitory hyperthyroidism can be seen in patients with viral treatment to take several tablets before a doctor's visit. This may thyroiditis. Most have had a recent upper respiratory tract be associated with a raised TSH, but normal free T4.
infection and present with neck tenderness and pain, which may Many patients with a history of differentiated thyroid cancer are advised to take suppressive doses of thyroxine. Guidelines4 Some patients have a low TSH but normal free T suggest that with persistent disease TSH should be kept below 0.1 mIU/L. Patients who presented with high-risk disease, but 3 can then be helpful as some patients will have T3 who are clinically free of disease, are advised to maintain TSH between 0.1 and 0.5 mIU/L for 5–10 years. Advice from normal values, but a persistently low TSH with a normal free commercial pathology laboratories that thyroxine doses be reduced in these patients should be resisted.
4 suggests autonomous thyroid function and a diagnosis of 'subclinical hyperthyroidism', which is usually associated with Adjusting treatment for hyperthyroidism
a nodular goitre (or, unusually, hypothalamic-pituitary disease). TSH may remain suppressed for weeks or even months after Subclinical hyperthyroidism in the elderly is associated with an a patient starts antithyroid medications. It is useful to monitor increased risk of atrial fibrillation, stroke and osteoporosis. free T4 and free T3 every 6–12 weeks to judge the adequacy of Hypothyroidism
treatment. A rise in TSH indicates overtreatment. Patients with severe hyperthyroidism may need more frequent monitoring.
A raised TSH and a low free T4 indicate primary hypothyroidism, almost always due to autoimmune thyroid Conclusion
disease but sometimes due to previous surgery or radioiodine Thyroid dysfunction is common in the general population and administration. The incidence of raised TSH and thyroid TSH measurements provide a sensitive method for detection. antibody levels and hypothyroidism increases with age and is An abnormal TSH requires further investigation, including at least measurement of free T4. Interpretation of the results It is not uncommon to find a raised TSH but normal free T4. of thyroid function tests is facilitated by an understanding of In most cases this suggests autoimmune thyroid disease. thyroid hormone physiology, especially the normal inverse This subclinical hypothyroidism is more likely to progress to relationship between free T4 and TSH concentrations. overt hypothyroidism when higher levels of TSH and thyroid Variations in assay performance mean that it may be helpful to consistently use the same laboratory for an individual Asymptomatic patients with a raised TSH and normal free T patient. An understanding of the effects of severe illness and require regular monitoring, especially if they are elderly or have medications on test results is also important.
high levels of antithyroperoxidase autoantibodies. Every six references
1. Canaris GJ, Manowitz NR, Mayor G, Ridgway EC. The There is considerable debate about the normal upper limit of Colorado thyroid disease prevalence study. Arch Intern Med the TSH reference range. The high background prevalence of autoimmune thyroid disease as well as the age, iodine status, 2. Ladenson PW, Singer PA, Ain KB, Bagchi N, Bigos ST, smoking prevalence and ethnicity of the 'normal' population has Levy EG, et al. American Thyroid Association guidelines for detection of thyroid dysfunction. Arch Intern Med raised the 'normal' upper limit. In people without these factors the upper limit is probably 2.5 mIU/L. While mildly raised TSH 3. Walsh JP, Bremner AP, Feddema P, Leedman PJ, Brown SJ, levels rarely require treatment, a concentration above 4.0 mIU/L o'Leary P. Thyrotropin and thyroid antibodies as predictors and the presence of thyroid antibodies is predictive of eventual of hypothyroidism: a 13-year, longitudinal study of a hypothyroidism and indicates that these patients need to be community-based cohort using current immunoassay techniques. J Clin Endocrinol Metab 2010;95:1095-104.
4. Cooper DS, Doherty GM, Haugen BR, Kloos RT, Lee SL, Adjusting thyroxine treatment
Mandel SJ, et al. Revised American Thyroid Association management guidelines for patients with thyroid nodules Replacement thyroxine in hypothyroid patients should be and differentiated thyroid cancer. Thyroid 2009;19:1167-214.
adjusted to maintain TSH at about 2 mIU/L. It takes about Conflict of interest: none declared six weeks for a change in thyroxine dose to achieve stable concentrations of free T4. Changes to the dose of thyroxine, and tests of thyroid function, should not be done more frequently, unless clinically indicated. It is not uncommon for patients who are less than optimally compliant with recommended thyroxine www.australianprescriber.com
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Source: http://www.gp.unimelb.edu.au/pctn/students/resources/tute7/ThyroidFunctionTestsAustPrescriber.pdf