More commonly, you may feel pain or discomfort during the blood draw. You may also bleed slightly after the needle is removed. You may develop a small bruise around the puncture site. A T4 test is a blood test that helps your doctor identify problems with your thyroid. This will ensure the most accurate interpretation of the test results. Learn why a thyroid-stimulating hormone test is performed, what to expect during the test, and what the test results may mean.
A T3 test is a blood test that helps your doctor determine if you have a thyroid problem. Here are some facts you should know about hypothyroidism and how it differs from hyperthyroidism. If you have symptoms of, or a history with thyroid disorders, it may make sense for you to check your TSH levels with an at-home TSH test. These are…. Health Conditions Discover Plan Connect.
T4 Test. Medically reviewed by Andrew Gonzalez, M. Why do doctors perform a T4 test? How do you prepare for a T4 test? Accordingly, in these assays, FT 4 measurements correlated only with serum albumin concentrations 7 Even then, the effects were not marked until albumin concentrations had fallen well below the reference interval for that protein [e.
Observed bias within the reference interval for albumin concentrations did not have significant consequences for diagnostic discrimination Over the next 20 years, continual improvements in the properties of the analogs eventually led to the abolition of their binding to serum proteins 64 Accordingly, there was no need to add albumin expressly to counteract bias from residual binding effects.
However, another problem manifested itself, which extra albumin in the assay ingredients helped to ameliorate. In patients receiving heparin injections, blood lipoprotein lipase is activated 72 When blood is withdrawn from such patients, NEFAs are produced in vitro, often to very high concentrations 74 75 76 77 The NEFAs in turn are capable of displacing substantial quantities of bound T 4 from the serum protein T 4 -binding sites and can increase the measured FT 4 considerably, often grossly out of the reference interval 30 31 74 75 76 77 Thus, assays containing albumin can be used with heparinized subjects with caution.
In severe NTI, there are additional problems. Initially, in the days of FTI supremacy, it was believed that without overt primary thyroid dysfunction, such patients were essentially euthyroid, although there were considerable decreases in the concentration of total T 4 into the hypothyroid range, not wholly corrected by FTI 40 41 42 79 Gold-standard methods for FT 4 measurement variously gave results below, within, and above the euthyroid reference interval 27 41 42 81 82 83 84 Simultaneously, thyrotropin TSH concentrations could also be low, normal, or occasionally above normal 86 87 88 89 , although not necessarily in harmony with the FT 4 values.
It was postulated that in NTI, the serum contained nondialyzable substances that could bind competitively to the T 4 -binding proteins, displacing bound T 4 into the free phase 90 91 This explanation rationalized how the gold-standard FT 4 measurements could be so out of line with total T 4 values, even when the serum binding protein concentrations especially transthyretin and albumin were also severely decreased.
Nevertheless, FT 4 estimates straddling the reference interval also create problems in discriminating true primary hypo- and hyperthyroidism in subjects with NTI from subjects with no apparent thyroid dysfunction. It was, no doubt, hoped that when new, more convenient assays were available, these anomalies would disappear.
In NTI, overlaying any general level of thyroid function, there seem to be continuous independent ebbs and surges in the production and retention of substances either directly or indirectly related to FT 4 as the disease progresses 81 82 83 84 85 86 87 88 89 90 91 These can include serum binding proteins 94 ; interfering substances, including administered drugs 32 33 ; TSH 90 91 92 ; and probably production of T 4 and its conversion to T 3 95 The true thyroidal state of the subject can thus be obscured by relatively short-lived changes in measured indicators, which may, at random, show up in single samples taken from a panel of NTI patients.
This highlights the danger of diagnosing NTI patients by single samples, making it difficult to compare assay performance if the panel of sera inevitably contains examples of these random events. Indeed, because of the tendency of FT 4 and TSH concentrations independently to enter and leave the euthyroid reference interval, it is recommended that both markers be measured in NTI and that careful monitoring is done to rule out effects that might alter the concentration of either marker, e.
Only in this way can we assess the underlying integrated level of thyroid function and discount transient anomalies. The difficulties in diagnosing hypothyroidism accompanying NTI have been vividly described Most studies comparing FT 4 assay performance in NTI have not taken this complexity fully into account and are generally poorly designed.
Assays with biases attributable to added albumin have often been criticized for giving below-normal values when TSH is presumably within the reference interval 31 32 However, because gold-standard assays lacking added albumin can equally give diagnostically meaningless above-normal values, misplaced confidence in their validity has often led to support for assays that show the same effect Here there is a logical inconsistency.
Conversely, if albumin-free assays give above-normal values when TSH is transiently low, a misdiagnosis of hyperthyroidism could equally be made. Neither methodology thus can be completely trusted in this situation. Additionally, albumin-containing assays may mitigate the transient effects on FT 4 of albumin-binding drugs in serum and depict the underlying thyroidal function more exactly. Finally, the potential effects of assay incubation on the concentration of interfering substances in serum should not be underestimated.
Albumin-containing assays can at least partially mitigate this effect The amount of exogenous albumin in the assay buffers that can be used to minimize interference in FT 4 assays depends very much on its properties of T 4 binding as well as its concentration.
It is well known that commercial sources of albumin can vary markedly in their T 4 -binding characteristics Such sources are properly termed preparations rather than well-defined purified compounds because denaturation of albumin can occur to various degrees during production.
However, a calculation can be made of how large a BP for added albumin will distort the FT 4 values of most sera to an acceptably minimal extent relative to the assay calibrators. Hence, FT 4 values are not grossly affected until endogenous albumin concentrations are less than half-normal and TBG concentrations are also substantially diminished.
Albumin preparations with weaker T 4 -binding affinities can be used in greater quantities because the smaller affinity offsets the effects of higher concentration in producing a suitable BP factor. Selecting assays that either do or do not contain albumin in their buffers appears to be a trade-off of intrinsic bias in FT 4 values downward in NTI and in all other situations where the binding capacity is reduced , caused by the added albumin, against increased sensitivity of an albumin-free assay to extraneous in vivo and in vitro effects.
Depending on their experience with discrimination of thyroidal disease in NTI subjects, assay users must decide for themselves which to use. A carefully controlled study on large numbers of subjects, with a defined and graded severity of systemic illness, is urgently needed, with full knowledge of medication taken and with multiple sampling, in which outcomes are known and where substantial numbers of NTI patients with primary hypo- and hyperthyroidism are also included.
Such a study, using an albumin-containing and an albumin-free assay for comparison, together with TSH measurements, would more usefully evaluate the sensitivity and specificity of each assay in diagnosing thyroid disease in these conditions.
In this way, one could offset the misdiagnosis frequency from the inevitable biases in the albumin-containing assay against the misdiagnosis frequency in albumin-free assays. It is unfortunate that albumin is the only useful and inexpensive protein able to mitigate the effects of interfering substances on FT 4 values in heparinized, drug-affected, or nonthyroidally ill patients. However, chemical modification of albumin may greatly attenuate or destroy its binding of T 4 without destroying its ability to bind other substances, such as NEFAs Judicious protein modification might produce a substance still able to quench interference without causing significant bias through extra T 4 binding.
This might yield an assay giving superior discrimination in NTI through nullification of the influence of interfering substances while not otherwise suffering from bias from albumin addition.
The performance of convenient assays for FT 4 has improved greatly since the first attempts FTI in Index methods were accepted until it became clear that they were still strongly influenced by TBG. Better assays, correcting more completely for TBG concentrations, forced a sometimes painful period of readjustment in this and other areas, which has taken 20 years or more to reconcile in some regions of the world.
By now, the better assays for FT 4 have eliminated the biasing effects that arose in the early assays from residual binding of labels to serum albumin. The most recent improvements have also largely addressed the remaining problem of T 4 autoantibodies in serum.
This is a problem that lies more in the vagaries of patient physiology than in any hope of a final, simple, and definitive answer in an ideal assay.
The scope for further improvement in FT 4 assay methodology is very limited. Robbins J, Rall JE. The iodine-containing hormones in blood. Hormones in blood : Academic Press London. Ekins R. Measurement of free hormones in blood [Review]. Endocr Rev ; 11 : 5 The free hormone hypothesis and measurement of free hormones [Editorial]. Clin Chem ; 38 : The free hormone hypothesis and measurement of free hormones [Letter].
Clin Chem ; 39 : Tabachnick M. Thyroxine-protein interactions. Thermodynamic values of thyroxine with human serum albumin. J Biol Chem ; : Prince H, Ramsden DB. A new theoretical description of the binding of thyroid hormones by serum proteins.
Clin Endocrinol ; 7 : Clin Chem ; 31 : A method for determining the free portions of substances in biological fluids. European Patent No. The direct estimation of free hormones by a simple equilibrium radioimmunoassay. The effect of serum dilution on free thyroxine FT 4 concentrations in the low-T 4 syndrome of nonthyroidal illness. J Clin Endocrinol Metab ; 61 : Validity of analog free thyroxin immunoassays.
Part II [Opinion]. Clin Chem ; 33 : Is free thyroxine accurately measured at room temperature?. Temperature effects of free thyroxine measurements: analytical and clinical consequences.
Free thyroxine assessed with three assays in sera of patients with nonthyroidal illness and of subjects with abnormal concentrations of thyroxine-binding proteins. Konno N. Serum thyrotropin response to thyrotropin-releasing hormone and free thyroid hormone indices in patients with familial thyroxine-binding globulin deficiency.
Endocrinol Jpn ; 23 : Roosdorp N, Joustra N. A numerical comparison of the use of T 3 -uptake values and of TBG levels for the estimation of free thyroxine in serum. Clin Chim Acta ; 98 : 27 Wilke TJ. A challenge of several concepts of free thyroxin index for assessing thyroid status in patients with altered thyroid-binding protein capacity.
Clin Chem ; 29 : 56 Clin Chem ; 35 : Clin Chem ; 34 : Protein bound T 4 dependence: the uncontrolled variable in free T 4 assays. Exp Clin Endocrinol ; : Variable underestimates by serum free thyroxine T 4 immunoassays of free T 4 concentrations in simple solutions. J Clin Endocrinol Metab ; 79 : Underestimates of serum free thyroxine FT 4 concentrations by free T 4 immunoassay.
J Clin Endocrinol Metab ; 79 : 76 Analytical performance of free and total thyroxine assays. Clin Chem ; 42 : Accuracy of free thyroxine measurements across natural ranges of thyroxine binding to serum proteins. Thyroid ; 10 : 31 Ellis M, Ekins RP. The radioimmunoassay of serum free triiodothyronine and thyroxine. Pasternak CA eds. Radioimmunoassay in clinical chemistry : Heyden London. Giles AF. An improved method for the measurement of free thyroxine in serum dialysates.
Clin Endocrinol Oxf ; 16 : Helenius T, Liewendahl K. Improved dialysis method for free thyroxin in serum compared with five commercial radioimmunoassays in nonthyroidal illness and subjects with abnormal concentrations of thyroxin-binding globulin. Clin Chem ; 29 : Larsen PR. Salicylate-induced increases in free triiodothyronine in human serum. Evidence of inhibition of triiodothyronine binding to thyroxine-binding globulin and thyroxine-binding prealbumin.
J Clin Invest ; 51 : You may need a thyroxine test if a family member has ever had thyroid disease or if you have symptoms of having too much thyroid hormone in your blood, a condition called hyperthyroidism , or symptoms of having too little thyroid hormone, a condition called hypothyroidism.
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes. You don't need any special preparations for a thyroxine blood test. If your health care provider has ordered more tests on your blood sample, you may need to fast not eat or drink for several hours before the test.
Your health care provider will let you know if there are any special instructions to follow. There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly. If your T4 test results are not normal, your health care provider will likely order more thyroid tests to help make a diagnosis. These may include:. Learn more about laboratory tests, reference ranges, and understanding results. Thyroid changes can happen during pregnancy.
Although it is not common, some women can develop thyroid disease during pregnancy. Hyperthyroidism happens in about 0. Thyroid hormone: How it affects your heart.
Updated September 17, The clinical use of thyroid function tests. Arq Bras Endocrinol Metabol. Laboratory testing in thyroid conditions - pitfalls and clinical utility. Ann Lab Med. Evaluation of the female patients with subclinical hypothyroidism by brainstem auditory evoked potentials: Case-control study. J Clin Diagn Res. American Thyroid Association. Thyroid function tests. Blood Test: T4 Thyroxine. Kids Health From Nemours website. Updated July T4, Free. Updated December 28, Thyroxine T4 Test.
National Library of Medicine Medline Plus website. Updated August Thyroid Function Tests. American Thyroid Association website. Updated Free T4. University of Rochester Medical Center website. Your Privacy Rights. To change or withdraw your consent choices for VerywellHealth.
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