LABORATORY TESTS FOR EVALUATION OF THYROID DISORDERS Maryam Tohidi Anatomical & clinical pathologist Research Institute for Endocrine Sciences
THYROID GLAND (15-25 gr), (12-20 gr), 2 lobes connected by isthmus, located anterior to the trachea.
Each lobe have about 20-40 follicles with central lumens containing: - Colloid (a protein reach material) - Thyroid hormones - Thyroglobulin (Tg) - Other glycoproteins. Thyroid cells: - Foillicular epithelial cells - & para-follicular C cells - Other non- parenchymal elements (Fibeoblasts, endothelial cells, lymphocytes, )
T3 RECEPTORS The effects of free thyroid hormones are mediated by T3 receptors located in the nucleous of cells. 4 isoforms of T3 receptor: 1, 2, 1 & 2. 1 & 1 on most tissues 2 is unique to the pituitary & some regions of hypothalamus (negative feed back)
STEPS OF THYROID HORMONE SYNTHESIS & SECRETION 1- Active transport of inorganic iodide into the cells by sodium- iodide symporter in the baso-lateral membrane (role of TSH & serum iodide level) 2- Iodination of tyrosine residues on Tg (a large glycoprotein synthesized by RER) to form MIT & DIT (oxidative reaction by TPO) 3- Coupling of iodotyrosine molecules within Tg to T4 & T3 (oxidative reaction by TPO) 4- Proteolysis (lysosomal degredation) of Tg with release of free iodotyrosine, T4 & T3 & secretion of iodothyronin in to the circulation
BOUND VERSUS FREE THYROID HORMONES 100% of circulating T4 is of thyroidal origin About 50% of T4 is monodeiodinated to T3 (5 position) & 40% to reverse T3 (5 position inner ring) 20%of T3 is of thyroidal origin & 80% produced enzymatically in non- thyroidal tissue (liver & kidney) by 5 - monodeiodination T4: 70% bound to TBG,20% transthyretin (prealbumin), 10% albumin T3: mostly bound to TBG Free T4 0.03 % of total T4 Free T3 0.3% total T3 Free hormones are biologically active.
ALTERATIONS IN TBG Increase: Pregnancy Genetic Acute & chronic hepatitis idiopathic Drugs: (Estrogens, OCP, Tamoxifen, Clofibrate, methadone, Heroin) Decrease: Liver failure Nephrotic Syn. Malnutrition idiopathic Genetic : complete or partial deficiency Drugs: (Androgens, Glucocorticoids)
TRH Serum TRH assay is not useful for evaluation & diagnosis of thyroid disorders because: - It is difficult to develop a specific Ab for immunoassay. - Low concentration in the peripheral circulation.
TRH STIMULATION TEST An intravenous bolus of 200 400 µg TRH is injected. Peak TSH secretion occurs 15 min after administration of exogenous TRH. Failure of TSH to rise means truly suppressed TSH level. The availability of the fourth generation of TSH assay (ICMA) has rendered the TRH stimulation test obsolete.
THYROID DISEASES Thyroid dysfunctions: Hyperthyroidism Hypothyroidism Thyroid enlargement or nodules Non-neoplastic lesions (Benign nodular goiter, hyperplasia, thyroiditis Neoplastic lesions - Benign - Malignant
THYROID FUNCTIONAL STATE Euthyroid state Subclinical hypothyroidism Overt hypothyroidism Subclinical hyperthyroidism Overt hyperthyroidism
LABORATORY TESTS FOE EVALUATION OF THYROID DISORDERS TSH T4 T3 Free T4 Free T3 T3 resin uptake Anti- TPO Anti- Tg Thyrogolbuline TSH receptor Ab-blocking Ab TSH receptor Ab TSI (previously known as thyroidstimulating immunoglobulin)
TSH A glycoprotein secreted by the pituitary alpha subunit (LH, FSH, hcg) & beta subunit Immunoassay for TSH: - First generation (1965) - Second generation(1980s- IRMA), sensitivity 0.1-0.2 µiu/l - Third generation, sensitivity 0.005 µiu/l
ELEVATED SERUM TSH LEVEL Hypothyroidism (most common cause) Rare causes: TSH-secreting pituitary tumor Thyroid hormone resistance Due to laboratory interference
SUPPRESSED SERUM TSH LEVEL Thyrotoxicosis * After treatment of hyperthyroidism (because TSH can remain suppressed for several months) During the first trimester of pregnancy (due to hcg secretion) In response to certain medications (e.g., high doses of glucocorticoids or dopamine). Factitious thyrotoxicosis
ASSAYS FORT3 & T4 & FREE HORMONES Immunoassay for Total forms Free hormones: - Physical separation of the unbound hormone fraction by ultracentrifugation or equilibrium dialysis - Immunoassay methods affected by illness or changes in binding proteins that are more resolved now)
T3 R-UPTAKE TEST The test is conducted by adding a known, excess amount of 125 I labeled T3 to a measured volume of serum. The added radiolabeled T3 binds to and saturates the unoccupied TBG sites and does not displace the more tightly bound T4. The excess, unbound 125 I T3 is then separated from the serum bound 125 I T3 by adding an adsorbent, such as ion exchange resin beads and centrifuging. The serum is decanted, the beads are washed and then counted in a scintillation counter.
Hyperthyroid cases exhibit a decreased concentration of unoccupied TBG sites and an increased T3 resin uptake. Hypothyroid patients have increased unoccupied TBG sites and decreased T3 resin uptake.
FREE THYROXINE INDEX (FT4I) Calculation of Free T4 Index = Total T4 x T3 uptake. example: T4 = 13.0, T-uptake= 29% FT4I: 13.0 X 29%= 3.77 (1.33-4.68) TSH= 3.2 (0.3-4.5) T4= 4.8 T- Uptake= 26% FT4I:4.8 X 26%= 1.25 (1.33-4.68) TSH= 5.6 (0.3-4.5)
THYROID FUNCTIONAL STATES Symptoms TSH (miu/l) free T 4 Overt hypothyroid yes > 10.0 decreased Subclinical hypothyroid no 4.5-10.0 normal Euthyroid no 0.45-4.5 normal Subclinical hyperthyroid no 0.1-0.45 normal Thyrotoxicosis yes < 0.10 increased
PRIMARY HYPOTHYROIDISM TSH: T4: T3: N or FT4: T3 RU: FT4I: anti TPO: N or anti Tg: N or
HASHIMOTO,S THYROIDITIS TSH: T4: N or T3: N or FT4: N or anti TPO: anti Tg:
GRAVES DISEASE TSH: T4: T3: FT4: T3 RU: FT4I: anti TPO: anti Tg: Tg: (Not needed for diagnosis & management)
NON- THYROIDAL ILLNESS Abnormal thyroid tests without underlying thyroid disorders. TSH: Variable T4: N or T3: FT4: Variable anti TPO: negative anti Tg: negative
CASE 1 A 32 y/o woman T4: 13.8 µg/dl (4.5-12.0) T3: 251 ng/ml (60-220) TSH: 2.48 µiu/ml (0.28-4.5) T-Uptake: 29 % (25-35) FT4I= (13.8 * 29)/100 = 4.0 (1.33-4.68)
CASE 2 A 32 y/o woman T4: 13.8 µg/dl (4.5-12.0) T3: 251 ng/ml (60-220) TSH: 0.18 µiu/ml (0.28-4.5) T-Uptake: 37 % (25-35) FT4I= (13.8 * 37)/100 = 5.11 (1.33-4.68)
CASE 3 A 58 y/o woman T4: 6.2 µg/dl (4.5-12.0) T3: 85 ng/ml (60-220) TSH: 8.43 µiu/ml (0.28-4.5) T-Uptake: 28 % (25-35) FT4I= (6.2 * 28)/100 = 1.74 (1.33-4.68) Anti- TPO= 356 IU/mL (up to 40)
CASE 4 A 73 y/o man T4: 4.1 µg/dl (4.5-12.0) T3: 62 ng/ml (60-220) TSH: 23.7 µiu/ml (0.28-4.5) T-Uptake: 23 % (25-35) FT4I= (4.1 * 23)/100 = 0.94 (1.33-4.68) Anti- TPO= 356 IU/mL (up to 40)
CASE 5 A 46 y/o man T4: 14.9 µg/dl (4.5-12.0) T3: 283 ng/ml (60-220) TSH: < 0.005 µiu/ml (0.28-4.5) T-Uptake: 35 % (25-35) FT4I= (14.9 * 35)/100 = 5.22 (1.33-4.68) Anti- TPO= 85 IU/mL (up to 40)
THYROGLOBULIN Normal individulas: up to 30 ng/ml (different expected values in various kits) Reflects thyroid mass, thyroid injury & TSH stimulation Increased in Graves disease, thyroiditis & large nodular goiter. Factitious thyrotoxicosis: undetectable Tg level. Clinically indicated only in follow- up of well differentiated thyroid cancers.
Thyroglobulin (htg) reflects thyroid mass Normal
THYROGLOBULIN AS A TUMOR MARKER For well differentiated thyroid cancers (PTC & FTC) When Anti- Tg is not positive. Case presentation
TRAB (STIMULATING & BLOCKING) Mainly thyroid stimulating Ab (TSAb) in Graves, disease Target of TSAb: TSH receptor TSAb mimic the action of TSH Thyroid blocking Ab is also present in Graves, disease but do not cause symptoms TRAb in 85 % of cases of Graves, disease Clinical utility??? TRAb assay used to predict the outcome of drug treated patients (lower titer before treatment, better chance for remission after 6-12 months of therapy) TRAb assay for prediction of thyroid dysfunction in neonates of mothers with Graves, disease.
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