In the name of God Dr.m.omidi Endocrinologist Assistant stant professor of medicine 25 APR 2012
Clinical evaluation of Thyroid function tests
Goal of TFT Evaluation of function of thyroid Monitoring of levothyroxine tx(either replacment or suppresive tx) Monitoring i of tx of hyperthyroidism Monitoring of thyroid malignancy(with ith TG and anti tg)
Thyroid function test Thyroid function tests included d five parameter 1.assay of HPT axis(with ith TSH) 2.concentration of T3 and T4 in serum 3.anti thyroid anti body 4.Tissue impact of thyroid hormone 5.throid iodine metabolism
TSH
TSH is a glycoprotein hormone composed of alfa and beta subunit. Alfa subunit similar il to other glycoprotein hormone such as FSH,LH,HCG. But beta subunit is specific for TSH and synthesis of this component is a rate limiting step in the synthesis of TSH.
TSH :stimulates synthesis and secretion of (T4) and (T3). Secretion of TSH is stimulated by (TRH), a hypothalamic h tripeptide. id TSH synthesis and release : regulated via a negative feedback mechanism by the level of thyroid hormones. Increased serum levels l of free T4 and T3 depress TSH secretion (hyperthyroidism), while decreased serum levels of free T4 and T3 result in excess TSH secretion (primary hypothyroidism).
TSH : no carrier protein TSH : unaffected by changes (such as liver or kidney failure, estrogens, and androgens) in proteins that are known to affect thyroid hormone carrier proteins
Average TSH levels l :.4 ± 4.44 mu/l, Alfa subunite similar to FSH,LH and HCG.normal serum level l of alfa is 1_5mic/L Increased in post monuposal,primary hypothyroidism and glycoprotein producing tumors of ant pituitary.
First generation immunoassy detected TSH above 1mu/lso can not deffrentiated between hyper and euthyroidism(nl lower limit of for TSH is.4) Second generation assay detected TSH above.1mu/l Third generation assay(chemiluminometric assay)can meassured TSH till.01
Second- and third-generation TSH assays can distinguish 50 40 euthyroid patients from hyperthyroid h patients. 3 rd generation assays 2 nd generation assays 1 st generation assays CV % In nterassay 30 20 10 0 FS = 0.01 0.02 mu/l FS = 0.1 0.2 mu/l Normal range FS = 1-2 mu/l 0.04 0.01 fourth-generation research assays (not yet available for clinical use) 0.001 to 0.002 mu/ml. 0.04 0.1 TSH mu/l 0.4 1 4 10
TSH Reference Ranges Serum TSH values : higher in neonates and children. Mean euthyroid TSH values: approximate to 1.5 miu/l, (range 0.3 to 4.0 miu/l in iodinesufficient populations)
Some advocated d that t some of pt with TSH level above 2.5 may developed hypothyroidism thus upper limt of normal range shoud be lowerd(those with posstive familial hx,posstive anti thyroid antibody,pregnancy and expouse to drug such as amiodarone,thyroid nodule)
But till TSH above 10 to15 no tx needed this recommendation is quistionable.
Current guidelines recommend : TSH as the first-line test This is more cost-effective than a panel approach (TSH + FT4 or FT4 + FT3) but necessitates the use of a TSH assay with a functional sensitivity below 0.02 mu/l i.e. (third generation). However, this TSH-first strategy can miss patients, gy p with central hypothyroidism or TSH secreting pituitary tumors who must be assessed by FT4
The first tests for assessment of thyroid function are TSH and free T4
CLINICAL UTILITY OF THYROID- STIMULATING HORMONE MEASUREMENT
3 rd gen. TSH assay elevated Within normal range suppressed below 0.3 mu/l FT4 Anti-TPO FT4 elevated FT4 normal R/o hypothyroidism caused by Hashimoto's thyroiditis R/O THYROTOXICOSIS FT3
Clinical situations associated ated with subnormal TSH values
Clinical situations associated with subnormal TSH values thyroid hormone excess endogenous hyperthyroidism or excess exogenous thyroid hormone points the activation of the negative feedback loop. Prolonged excessive thyroid hormone physiological "atrophy" of the thyroid stimulatory limb of the hypothalamic-pituitary thyroid axis. Thus several months are usually required for the reestablishment of TSH secretion after the relief of thyrotoxicosis. After excess thyroid hormone treatment after the transient hyperthyroidism associated with subacute or some variants of autoimmune thyroiditis Shorter suppression Severe illness
rt3 Hormo one concen ntration FT4 TSH Normal Range TtlT4 Total T4 T3 Mild Moderate severe Recovery Stage of disease TFT during NTI
Clinical situations associated with Points subnormal TSH values High-dose glucocorticoids Exogenous dopamine Pharmacological amounts of retinoids Acutely but not chronically In critically ill patients, this effect of dopamine can be superimposed on the suppressive ssi effects of acute illness on thyroid function, reducing T4 production to even lower levels. Dopamine is sufficiently potent to suppress TSH to normal levels in sick patients with primary hypothyroidism. severe central hypothyroidism associated with very low serum TSH concentration has been reported in patients with cutaneous T-cell lymphoma treated with high-dose retinoid X receptor-selective ligand able to suppress TSH secretion.
hcg acute psychosis depression agitated psychoses Congenital central hypothyroidism a thyroid stimulator; In normal pregnancy as a slightly subnormal TSH during the first trimester (0.2-0.4 mu/l) ; frank, though mild, hyperthyroidism in patients with choriocarcinoma or molar pregnancy. may have high thyroid hormone levels and suppressed or elevated TSH values. Etiology,unkown low serum TSH; may result from mutations affecting TSH alpha gene or the Pit-1 gene
In normal pregnancy TSH suppressed in first trimester and return to Nl in second and third trimester. Persistent suppression after first trimester(bellow.1) suggested that hyperthyroidism y due to autonomous thyroid function.
Clinical situations associated ated with elevated TSH values
Clinical situations associated with elevated TSH values Primary hypothyroidism points The serum free T4 is low normal or reduced but the serum free T3 values remain normal until the level l of thyroid function has markedly deteriorated. Iodine deficiency(the most common cause worldwide) Western Europe, South America, Africa and Asia. during the recovery phase A transient effect ; after a severe illness In such patients a "reawakening" of the hypothalamic-pituitary thyroid axis occurs with the improvement in their clinical state.
The remainder of the conditions associated with an elevated TSH are extremely rare : partial or complete TSH resistance inactivating point mutations of the TSH receptor gene. In a patient who has an elevated serum FT4, the presence of TSH at normal or increased levels should lead to a search for either resistance to thyroid hormone or a thyrotroph tumor. Hypothalamic-pituitary dysfunction normal or even modest increases in TSH are explained by the lack of normal TSH glycosylation in the TRH-deficient patient. The diagnosis is generally made by finding a serum free T4 index which is reduced to a greater extent than expected from the coincident serum TSH. Psychiatric illness either elevated or suppressed TSH, but the abnormal values are not usually in the range normally associated with symptomatic thyroid dysfunction
Addison's s disease The effect of glucocorticoids to suppress TSH secretion; TSH may be slightly elevated in the absence of primary thyroid disease. the presence of endogenous antimouse gamma globulin antibodies These antibodies, like TSH, can complex the two TSH antibodies resulting in artificially elevated serum TSH assay results in euthyroid patients; forming false bridges between the solid phase and signal antibodies.
Total T3 and T4 99/ of these hormone bound to circulating proteins including TBG,albumin and TTR Refrence range for T4(5_11mic/dl) g and T3(70 _190ng/dl)
Because high h level l of these hormone are bound to binding proteins,only measurement of total hormone is misleading Most common cause of change in total T3 and T4 is change in protein binding and is not due to thyroid problem. Binding proteins included TBG, albumin and TTR.
transport TBG TTR/TBPA Albumin T4 68% 11% 20% 99.97% T3 80% 9% 11% 99.7%
Factors that increased TBG 1.estrogen(pregnancy,ocp.HRT,tamoxifen heroin, methadone) 2.acute and chronic hepatitis 3.perphenazine,HIV ph, interferon n, genetic factor Factor that decreased TBG 1.androgens,danazol,glucocorticoid,RGH) g 2.nephrotic syndrome 3.acromegaly 4.Cirrhosis 5.Major systemic illness, Asparaginase
For evaluation the effect of binding protein on hormone assay T3Ru should be measured Normal range of T3Ru (27_33/) In condition of binding protein problems, level of T3 and T4 inversely related to T3Ru. T3Ru used for estimation of FT4Ior FT3I FT4i=T3RU total T4
Ru mused for estimation of ft4i
1.TBG increased,tt3 and TT4 increased, T3Ru decreased TBG decreased,tt3 and TT4 TBG decreased,tt3 and TT4 decreased,t3ru increased
In condition that thyroid pathology present level T3 andt4 related to T3RU(in hypothyroidism h both decreased and in thyrotoxicosis both increased)
Effect of various medication on TFT
Decreased TSH glucocorticoid Dopamine Octreotide bexarotene
Decreased secretion of thyroid hormone Amiodarone Iodine lithium
Estrogen Tamoxifen Heroin Methadone HRT Increased TBG
decreased TBG Androgens Anabolic steroids glucocorticoids
Illness and thyroid Acute illness affected TFT strongly So recommended that TFT dose not evaluated in acute ill patients if thyroid dysfunction is not high suspicious.
In acute illness conversion of T4 to T3 decreased Binding of thyroid hormone to binding protein is altered. Secretion of TSH decreased Various medication used that affected thyroid function
In mild illness such as pneumonia and UTI,TT3 decreased but total and free T4 and TSH is normal(t3 syndrome)
As underlying disease progress to moderate degree of severity TSH decreased d but TT4 is normal and RT3 is elevated.
In sever disease and in critical ill patients T4 decreased(t4 syndrome) during recovery phase of illness TSH elevated but still T4 and T3 are low(like hypothyroid )
About six weeks needed d after complete improvement of underlying disease for normalization of TFT.
rt3 Hormo one concen ntration FT4 TSH Normal Range TtlT4 Total T4 T3 Mild Moderate severe Recovery Stage of disease TFT during NTI
Free Hormone Tests (FT4 and FT3) free hormone hypothesis: free fraction of hormone is responsible for the biologic activity of thyroid hormones at the cellular level
Free T4 and T3 Measured with dialysate or ultrafiltrate(the accurate method). Only.3 of T3 and.02of T4 are free. In pregnant and ill pt free hormones do not accurate so for pregnancy the normal range of T4 multiplied by 1.5in lieu of a free T4 assay. In severely ill Pt free T4 falsely low and FT4I is better for asses of thyroid function Three method used(1)two step labeled hormone (2)one step (3labeld antibody approach All subject to artifacts from endogenous antibody,abnl binding protein or illness
Free Hormone Tests (FT4 and FT3) Index Methods Free T4 Index (FT4I) Free T3 Index (FT3I) Absolute Methods Equilibrium Dialysis Ultrafiltration Gel Filtration/Adsorption ti ti chromatography h Comparative Immunoassays Two-Step labeled Hormone One-step Labeled Hormone Analog Labeled Antibody
Index Methods (FT4I and FT3I) require two separate measurements: TT4 or TT3 Thyroid Hormone Binding Ratio (THBR) or "Uptake test Free Hormone Indexes (FT4I and FT3I): simple calculations approximation of the free hormone concentration nt ti n in the presence of abnormal binding proteins estimate free hormone concentrations: 40 y
FT4I and FT3I Current THBR tests are usually able to produce normal FT4I and FT3I values when TBG abnormalities are mild (i.e. pregnancy and estrogen therapy) these tests often fail to normalize FT4I and FT3I values in euthyroid patients with grossly abnormal binding proteins (congenital TBG extremes, familial dysalbuminemic hyperthyroxinemia (FDH), thyroid hormone autoantibodies and NTI
Clinical Performance Free Hormone Tests The indirect index tests (FT4I) and the direct comparative methods are all protein dependent to some extent These binding protein abnormalities cause discordances d between the total t and free hormone concentrations
Summary of a two-step serum free T4 Summary of a two step serum free T4 immunoassay
One-Step, Labeled Hormone-Analog FT4 and FT3 Methods hormone-analog n methods were pinipll principally engineered to give normal FT4 values in high TBG states They were found to have poor diagnostic accuracy in the presence of abnormal albumin concentrations secondary to Familial Dysalbuminemic Hyperthyroxinemic (FDH), NTI, high FFA concentrations or thyroid hormone autoantibodies
Clinical Performance Free Hormone Tests The only reason to select a free hormone method (FT4 or FT3) in preference to total hormone is to improve the diagnostic accuracy in patients with thyroid hormone binding abnormalities
Clinical Performance Free Hormone Tests (continue) No current FT4 method is universally valid in all clinical situations Common conditions that decrease the diagnostic accuracy of current free hormone tests in ambulatory patients include: TBG abnormalities (TBG excess or deficiency) Familial Dysalbuminemic Hyperthyroxinemia T4 and T3 autoantibodies interfering substances such as Rheumatoid Factor and Heterophilic antibodies (HAMA)
Anomalous Protein Binding of Tracer familial l dysalbuminemic i hyperthyroxinemia (FDH): falsely high free T4 estimate if serum albumin is subnormal, free T4 estimates by analogue tracer methods tend to be low In renal failure; analogue assays: low serum free T4 ( 40% in predialysis samples)
FREE T4 IN SPECIAL CIRCUMSTANCES Pregnancy Thyrotoxicosis and Hypothyroidism y Effect of Competitors for Thyroid Hormone Binding The Heparin Artifact Critical Illness
Pregnancy Interpretation of measured serum free T4 values during pregnancy is complicated by methodologic differences free T4 and free T3 decrease in the second and third trimesters, with mean levels reduced about 20% to 40% below the normal mean but subnormal levels are uncommon Roti and co-workers compared free T4 by seven different commercial methods in 23 euthyroid women and their infants at term, they found wide method-dependent variations
Pregnancy (continue) There is growing concern that the lack of method-specific, trimester- specific reference ranges for current FT4 tests renders these tests unreliable for assessing thyroid status during pregnancy TT4 measurements may be a better option if the reference limits are adjusted d for increased TBG by a factor of 1.5
Critical Illness Estimation of thyroid function during critical illness is influenced by multiple medications, such as dopaminergics, furosemide, and glucocorticoids some free T4 methods are vulnerable to the effects of NEFA, which may increase in vitro in heparin-treated patients Lack of specificity of free T4 and TSH results is the basis for recommending against routine testing of thyroid function during critical illness
Critical Illness there is a good argument for preferring measurement of total T4, a parameter that is generally free of assay artifact, versus free T4 during critical illness
Thyrotoxicosis and Hypothyroidism FT4 and FT3 do not maintain a linear relationship with the total hormone levels in thyrotoxicosis or hypothyroidism. In severe thyrotoxicosis, the total concentration of T4 can be two- to threefold elevated potential rise in free T4 is "buffered" by increased binding to TTR and albumin, there is a disproportionate increase in free T4 as total T4 increases.
Effect of Competitors for Thyroid Hormone Binding Numerous medications can displace T4 and T3 from TBG, but it is technically difficult to obtain an accurate reflection of these effects with current methods of measuring free T4
PRINCIPAL DRUGS THAT DISPLACE T4 FROM THYROXINE- BINDING GLOBULIN IN NORMAL HUMAN SERUM Drugs Salicylates Acetyl salicylic acid Salicyl salicyclic acid Furosemide Fenclofenac Mefenamic acid Flufenamic acid Diclofenac Difunisali Phenytoin Carbamazepine Mean Percent Increase in Free T4 Fraction 62 >100 5-30 90 31 10 7 37 45 30
Artifact The effect of heparin in increasing serum free T4 is an important in vitro phenomenon that can lead to spuriously high estimates of circulating free T4
3 rd gen. TSH assay elevated Within normal range suppressed below 0.3 mu/l FT4 Anti-TPO FT4 elevated FT4 normal R/o hypothyroidism caused by Hashimoto's thyroiditis R/O THYROTOXICOSIS FT3
TSH(low) TSH (high h 5-10) Subclinical hypo T4,ANTI TPO Low T4, NL T4,poss anti tpo Or pregnancy or infertility NL T4,neg anti tpo TX TX f/u 6-12 m
T3RU and THBR
Effect of amiodarone on TFT Decreased release of T3 and T4 Decreased T4 to T3 conversion Decreased synthesis of thyroid hormone in suspected patients(hypothyroidism)
Thyroid inflammation(thyroiditis) or type 2 amiodarone induced thyrotoxicosis(ait) Hyperthyroidism or type 1 AIT
Biochemical cal markers of thyroid function 1.Thyrotoxicosis i increased(osteocalcin,alkp,shbg,ferritin,v wf,anf) fa f) Decreased(LDL,Lpa) 2.Hypothyroidism Increased(LDL,LPa,norepinephrine,CK) Decreased(vasopersin)
Effect of pregnancy on thyroid 1.Urinary iodine excretion increased so need for iodine increased(150-200mic/day) 2.TBG secretion increased so level of total T4 and T3 increased but free level are nl 3.Autoimmune thyroid diseases during pregnancy ameliorated but after pregnancy may be exacerebrated 4.During first trimester due to HCG effect TSH suppressed and in 2 nd and3th trimester normalized.
So if after 1 st trimester t TSH still suppress autonomus hyperfunctioning of the thyroid gland cause of abnl thyroid TFT Thyroid function test should be normal in hypothyroid h women when planed pregnancy. During pregnancy need for levothyroxine 25-5050 percent increased.
Nuclear medicine and thyroid 1.RAIU:done with iodine(125 or 131) and 2 and 24 hr uptake measured. Used for deffrentiation of cause of thyrotoxicosis and dosimetry 2.Thyroid scan:done with Tc99 or I(125 and 131). Used for evaluation of thyroid nodule when TSH is suppressed, f/u of thyroid malignancy and cause of thyrotoxicosis. Nuclear imaging of thyroid contraindicated in pregnancy. 3.Thyroid ablation with I131 for tx of hyperthyroidism and thyroid malignancy.
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