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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
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
(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
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.
| VoLUMe 34 | NUMBer 1 | feBrUArY 2011
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 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
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
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.
| VoLUMe 34 | NUMBer 1 | feBrUArY 2011
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
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
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.
Common results of thyroid function tests
Hyperthyroidism (consider Graves', measure TSH receptor autoantibodies)
Subclinical hyperthyroidism (consider nodular thyroid disease)
| VoLUMe 34 | NUMBer 1 | feBrUArY 2011
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
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
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
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
| VoLUMe 34 | NUMBer 1 | feBrUArY 2011
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