Clinical Chemistry
Last revised 14 February 2008
Last major update July 2007
Copyright (c) 2007-8, PathologyOutlines.com, Inc.
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Disclaimer: this information is intended for pathologists and laboratory personnel, who understand that test interpretation must be made in the context of clinical history using reasonable medical judgment.
Clinical Chemistry - table of contents
Thyroid related: calcitonin, calcium stimulation test, free thyroxine index, hyperthyroidism, hypothyroidism, nonthyroidal illness, pentagastrin stimulation test, reverse T3, screening-adult thyroid disorders, screening-neonatal hypothyroidism, screening-pregnancy, T3 receptors, T3 uptake, thyroglobulin, thyroglobulin antibody, thyroid binding globulin, thyroid physiology, thyroid stimulating hormone (TSH), thyroid stimulating hormone receptor antibody, thyroperoxidase antibody, thyrotropin releasing hormone (TRH), thyroxine (T4), thyroxine-free (free T4), triiodothyronine (T3), triiodothyronine-free (free-T3), urinary iodide
Primary references for Clinical Chemistry
American Journal of Clinical Pathology, (AJCP), January 1975 to July 2007
Archives of Pathology and Laboratory Medicine (Archives), January 1976 to June 2007
Biomed Center, 1 March 1997 to 29 June 2007
Henry’s Clinical Diagnosis and Management by Laboratory Methods (2006, 21st ed)
Websites: ARUP Laboratories, Specialty Laboratories
Journal search terms: each disease entity listed
Please refer to these primary references for more detailed discussions and additional images
Thyroid related tests
Also known as thyrocalcitonin
Physiology: (1) preprocalcitonin (141 amino acids) is produced by parafollicular C cells of thyroid gland; (2) peptide cleavage leads to procalcitonin (116 amino acids), which contains immature calcitonin (33 amino acids); (3) peptidylglycine-amidating mono-oxidase enzyme produces mature calcitonin (32 amino acids)
Function: calcitonin inhibits osteoclasts from resorbing bone, leading to decreased serum calcium (antagonized by parathyroid hormone); levels are controlled by serum calcium
Diagrams: calcitonin production; calcitonin and parathyroid hormone
Methodology: immunometric assays using monoclonal antibodies to N terminal region and C terminal region; results vary by methodology (Tumori 2003;89:566, Clin Chem Lab Med 2007;45:662)
Indications: (a) medullary thyroid carcinoma-to detect residual tissue or metastases after thyroidectomy (get baseline prior to surgery and also measure 6 months post surgery; long doubling times predict good prognosis (J Clin Endocrinol Metab 2005;90:6077); (b) may help monitor other neuroendocrine carcinomas that produce calcitonin
Limitations: medullary thyroid carcinoma is rarely calcitonin negative (World J Surg Oncol 2006;4:97); high levels in newborns should be repeated several weeks later to see if they drop; run pentagastrin stimulation test if calcitonin value > 10 ng/L but not clearly diagnostic
Reference ranges: adults: < 10 ng/L (pg/ml) indicates no medullary thyroid carcinoma
Suggested reference ranges for Advantage ® system: (Clin Chem 2004;50:1828)
Children less than 6 months: < 40 ng/L; 6 months to 3 years: < 15 ng/L; older children same as adults
Adult women: < 5 ng/L; adult men: <12 ng/L
High values: medullary thyroid carcinoma (> 100 ng/L), C cell pathology (in MEN2 patients), neuroendocrine tumors, hypergastrinemia, hypercalcemia
References: Wikipedia
Calcium stimulation test for calcitonin
A provocative test to expose C cell abnormalities, since it increases calcitonin at all stages of medullary thyroid carcinoma
Indications: has been replaced by genetic testing; may be less sensitive than pentagastrin stimulation test
Procedure: give 2.5 mg/kg of calcium gluconate over 30 seconds, collect blood at time 0, 1, 2 and 5 minutes to determine calcitonin levels
Interpretation: rise in calcitonin > 100 ng/L suggests early medullary thyroid carcinoma
Also called T7 index (since measurement involves T3 and T4)
Indications: quantitate pituitary resistance to thyroid hormone (J Clin Endocrinol Metab 2000;85:2299); previously the primary method to estimate free T4 (now measure free T4 directly) or screen for thyroid dysfunction (now use TSH)
Calculation: FTI = total T4 x T3 uptake / mean T3 uptake of reference laboratory, or total T4 x serum binding correction factor
Interpretation: usually increased during hyperthyroidism, decreased during hypothyroidism; attempts to correct for changes in total thyroxine caused by changes in thyroid binding globulin
Limitations: nonspecific, low results also associated with non-thyroidal illness; free T4 is more accurate in women taking oral contraceptives (Clin Nucl Med 1981;6:168) and in general
See also Hyperthyroidism in Thyroid chapter
Hyperfunction of gland (usually elevated levels of free T3/T4, but see subclinical hyperthyroidism below)
Thyrotoxicosis refers to clinical effects of free thyroid hormone; source may not be thyroid gland
May be masked by nonthyroidal illness syndrome (J R Soc Med 2003;96:185)
Diagnosis: high free T4; TSH may be low (primary hyperthyroidism) or high (secondary hyperthyroidism); in subclinical hyperthyroidism, T3 and T4 are normal but TSH is low
Primary hyperthyroidism: intrinsic thyroid gland abnormality producing excess T4; high free T4, low TSH, normal TRH stimulation test
Secondary hyperthyroidism: abnormal TRH stimulation test (problem is at level of pituitary, including TSH producing tumors), high TSH, high free T4
Subclinical hyperthyroidism: low TSH (< 0.1 µIU/ml), normal T3 and T4 (Eur J Endocrinol 2005;152:1), no clinical hyperthyroidism; may be due to exogenous thyroid (Hormones (Athens) 2006;5:119); patients have increased risk of atrial fibrillation, cardiac death and osteoporosis
T3 hyperthyroidism: 1-4% of hyperthyroid patients; low TSH, high free T3, normal free T4; associated with early treatment of hyperthyroidism with antithyroid drugs
T4 hyperthyroidism: high T4, normal T3; due to primary hyperthyroidism causes, also iodine, amiodarone (Archives 2003;127:e275), pregnancy (2%)
Causes: Graves’ disease (85%-circulating autoantibodies to TSH receptor); overdose of exogenous thyroid hormone, hyperfunctioning multinodular goiter or thyroid adenoma, thyroiditis, struma ovarii, iodide ingestion, choriocarcinoma/hydatidiform mole, maternal Graves’ disease, pituitary adenoma; table of causes
Treatment: beta blockers for symptoms, thionamide-type drugs to block new hormone synthesis, iodine to block release of T4/T3
References: Am Fam Physician 2005;72:623, Med J Aust 2004;180:186, eMedicine #1, #2
See also Hypothyroidism in Thyroid chapter
May initially have transient hyperthyroidism
May cause macrocytic, nonmegaloblastic anemia with normal RDW
Primary hypothyroidism: due to destruction or ablation of thyroid gland (surgery, radiation, Hashimoto’s thyroiditis, developmental), interference with thyroid hormone synthesis (idiopathic, genetic [J Med Genet 2005;42:379], drugs [lithium, iodide, methimazole, PTU]), iodine ingestion (Intern Med 2007;46:391) or iodine deficiency; T4 is low causing high TSH
Other causes: chronic renal failure has normal TSH, low T3 and T4 (Nucl Recept 2005;3:1), bromide intoxication (AJCP 1988;89:802)
Secondary hypothyroidism: pituitary disorder causes reduced TSH secretion
Tertiary disease: hypothalamic lesion causes reduced TRF secretion
Subclinical hypothyroidism: high TSH, normal T3 and T4; no clinical symptoms of hypothyroidism (Am Fam Physician 2005;71:1763)
Cretinism: hypothyroidism developing during infancy/childhood; may be due to maternal hypothyroidism (maternal T3/T4 crosses placenta and is critical to fetal brain development before fetal thyroid gland develops); now rare due to newborn testing and iodine supplementation (Pediatrics 2006;117:2290)
Myxedema: chronic hypothyroidism in older child or adult
References: Wikipedia, eMedicine #1; #2
Nonthyroidal illness affecting thyroid function tests
Also called euthyroid sick syndrome or low T4 syndrome
Seriously ill patients (sepsis, myocardial infarction-Angiology 2002;53:699) may have abnormal thyroid function tests without thyroid pathology
Due to reduced serum iodothyronine deiodinases (J Clin Endocrinol Metab 2003;88:3202)
Often caused by malnourishment (JPEN J Parenter Enteral Nutr 2006;30:415)
Initially these patients have low total T3 and free T3; later thyroid binding globulin is disrupted causing low total T4
TSH is initially low to low-normal, then rises to normal or above normal as illness resolves, then returns to normal levels
TSH is suppressed by glucagon, dopamine and high doses of corticosteroids, which complicates interpretation
Common in medical intensive care unit (44%), and associated with poorer prognosis (Metabolism 2007;56:239)
Diagrams: in critical illness, inhibition of 5’ deiodinase causes reduced T3 and increased reverse T3 levels
References: J Clin Endocrinol Metab 1999;84:151, eMedicine
Indications: detect residual medullary thyroid carcinoma or other parafollicular C cell (thyroid gland)l pathology (Surgery 1999;126:1089, J Clin Endocrinol Metab 2004;89:515); intraoperative testing may predict lateral neck lymph node involvement (Surgery 2007;141:166)
Note: also measures gastric gastrin release and resulting pH changes (Urol Int 2003;70:178)
Methodology: give 0.5 µg/kg body weight of pentagastrin IV over 5 minutes, collect blood at time 0, 1, 2, 5 and 10 minutes and measure calcitonin levels
Interpretation: 50-100 ng/L indicates possible medullary thyroid carcinoma or other pathology; >100 ng/L indicates probable medullary thyroid carcinoma (J Clin Endocrinol Metab 2007;92:450)
Baseline or post-pentagastrin values > 10 ng/L indicates C cell pathology in MEN2 or residual tissue in medullary thyroid carcinoma patients after surgery
Limitations: elevated calcitonin after testing in patients with pseudohypoparathyroidism (Exp Clin Endocrinol Diabetes 2002;110:86), neuroendocrine carcinoma of foregut (Langenbecks Arch Surg 2000;385:398)
Adverse effects: substernal discomfort, nausea, vomiting, metallic taste, abdominal cramping, flushing, warmth, urgency, esophageal spasm, tachycardia, headache (Clin Chem 2002;48:1505)
Major metabolite of T4; 40% of T4 is 5 deiodinated (by 5-deiodinase) peripherally to form reverse T3
No biologic activity
Half life is 4 hours (short)
Bound to thyroid binding globulin
Wilson’s syndrome: questionable entity with no supporting publications (to my knowledge) of reverse T3 dominance causing hypothyroidism and low body temperature
Indications: not routinely measured; serum levels correlate with severity of hyperemesis gravidarum (Arch Gynecol Obstet 2000;264:57)
High levels: nonthyroidal illness (have reduced total serum T3), healthy newborns (J Clin Invest 1975;55:1137), hyperthyroidism, use of amiodarone, propranolol or estrogen, pregnancy (J Clin Endocrinol Metab 1977;44:660), fasting (Thyroid 1992;2:11)
Screening-adult thyroid disorders (not cancer)
See also screening-pregnancy below
The US Preventive Services Task Force concludes that evidence is insufficient to recommend for or against routine screening for thyroid disease in adults (Am Fam Physician 2004;69:2415, Ann Intern Med 2004;140:128)
American Thyroid Association recommends screening every 5 years beginning at age 35, particularly in women (Arch Intern Med 2000;160:1573)
High incidence of unrecognized thyroid disease in elderly (Recenti Prog Med 2004;95:308)
Recommended screening test is TSH
Abnormal values should be confirmed with a fresh specimen
Screening-neonatal hypothyroidism
See also screening-pregnancy below
Incidence: neonatal hypothyroidism occurs in 1 per 3 to 5,000 births, increased risk (up to 1 per 1,000 births) with multiple pregnancies (J Clin Endocrinol Metab 2007 May 8; [Epub ahead of print], Eur J Endocrinol 2005;153:765) and in iodine-deficient areas (Pediatr Int 2007;49:76); transient hypothyroidism of prematurity is associated with chorioamnionitis (J Perinat Med 2005;33:514)
Rarely due to maternal autoimmune thyroid disease (Hormones (Athens) 2003;2:113)
Low maternal free T4 in first trimester, even without symptoms, may cause neurodevelopmental difficulties in newborns (Pediatrics 2006;117:161)
Symptoms (if no laboratory screening): hypothermia, constipation, jaundice, poor feeding, hoarse cry, macroglossia, hypoactivity (Bosn J Basic Med Sci 2005;5:26); also extrathyroidal congenital abnormalities (Horm Res 2007;68:272)
Indications: routinely performed on all newborns; screening prevents severe mental retardation if treatment begins within 1 month of birth; mild hypothyroidism may cause cardiovascular mortality or damage to offspring of pregnant women
Methodology: use dry blood spots or cord serum to measure both T4 and TSH
If measure only T4 (checking for low values), have high false positive rate due to prematurity or congenital absence of thyroid binding globulin, and miss patients with elevated TSH but normal T4
If measure only TSH (checking for values of 10 mIU/L or higher), have false positives from premature or severely stressed infants and miss infants with hypothalamic or pituitary disease
Algorithm (flowchart):
Very low birth weight infants (J Med Screen 2005;12:166) and dizygotic twins (An Pediatr (Barc) 2006;65:129) should be retested at 2 weeks and 4-6 weeks to detect late onset hypothyroidism
TSH less than 10 mIU/L - no further action for newborns
TSH 10-20 mIU/L - retest at 2-6 weeks
TSH > 20 mIU/L - endocrine workup needed
Treatment: thyroxine (not mixed with soy [Arch Dis Child 2004;89:37, Endocr Pract 2001;7:193] or iron [South Med J 1997;90:637], which inhibit absorption) and close followup by endocrinologist (J Perinat Med 2005;33:543); may be able to discontinue if clinical and laboratory values are normal after 2-3 years of followup (Zhejiang Univ Sci B 2005;6:1206; treatment usually associated with normal subsequent growth (Pediatr Int 2007;49:443) but intellectual defects may persist (J Clin Endocrinol Metab 2006;91:418)
References: Pediatrics 2006;117:2290, J Pediatr Endocrinol Metab 2006;19:1291, J Clin Endocrinol Metab 2006;91:3370, eMedicine
Screening-pregnancy related thyroid disorders
Overt thyroid disease is present in 1% of pregnant women, 2-3% have subclinical hypothyroidism, 10-15% have positive anti-thyroid antibodies
All pregnant and lactating women require additional iodine intake; monitor with urinary iodine concentration
Hypothyroidism should be corrected before initiation of pregnancy and euthyroidism maintained throughout (Ther Drug Monit 2006;28:431); however low T4 with normal TSH may not affect perinatal outcome (Obstet Gynecol 2007;109:1129)
Therapy also recommended for subclinical maternal hypothyroidism, although risk to fetus is unknown
Maternal hyperthyroidism (Graves’ disease-Nat Clin Pract Endocrinol Metab 2007;3:470) can cause fetal hyperthyroidism (due to transplacental TSH-receptor antibodies); also associated with maternal hypothyroidism due to anti-thyroid drugs
Maternal autoimmune thyroid disease is associated with increased risk of miscarriage (J Coll Physicians Surg Pak 2006;16:468); these women should have TSH monitored for increased levels
Postpartum thyroiditis occurs in 2-5% (Obstet Gynecol 2006;108:1283)
Thyroid function screening recommended for women with history of thyroid disease (Gynecol Endocrinol 2007;23:138), family history, goiter, thyroid antibodies or autoimmune disorders, signs/symptoms suggestive of thyroid disease, type I diabetes, prior head and neck radiation, history of miscarriage or preterm delivery (J Clin Pathol 2005;58:449)
However, up to 1/3 of women with hypothyroidism would be missed by targeted screening (J Clin Endocrinol Metab 2007;92:203)
References: J Clin Endocrinol Metab 2007 Jun 3; [Epub ahead of print], National Guideline Clearinghouse
4 isoforms: alpha1, alpha2, beta1, beta2
Alpha1, Alpha2 and Beta1 receptors are present in most tissue, bind T3 to promote protein synthesis
Beta2 receptor is unique to anterior pituitary gland, hypothalamus and developing ear; mediates negative feedback of TSH (Mol Endocrinol 2005;19:1529); mutations cause thyroid hormone resistance with varying degrees of hypothyroidism
Alpha2 receptor is inhibitory
References: Colorado State University, OMIM 190120 (alpha1), 190160 (beta)
Also called T3 or T4 resin uptake or Thyroid Hormone Binding Ratio (THBR)
Measures proteins, not thyroid hormones
Relative measurement of unoccupied thyroid hormone binding sites on circulating proteins, including thyroid binding globulin (TBG)
Should be reported as a ratio to normal serum
Indications: previously used to calculate free thyroxine index (total T4 x T3 uptake), an estimate of free T4 (now free T4 is ordered directly); attempts to correct for changes in thyroid binding globulin due to liver disease, protein losing states, pregnancy or various drugs
Method: add labeled T3 (or T4) plus resin (a secondary binder) to serum; amount of labeled T3 bound to resin is measured by chemiluminescence; more labeled T3 is bound to resin if more thyroid binding globulins are occupied by thyroid hormone
Recommended to be based on the ratio between absorbent counts divided by the total minus absorbent counts, rather than the ratio between absorbent counts and total counts (see guideline 12)
Interpretation: high with hyperthyroidism, low with hypothyroidism
Limitations: invalid if other proteins or immunoglobulins compete with TBG, including familial dysalbuminemic hyperthyroxinemia (J Clin Endocrinol Metab 2000;85:2786, Thyroid 2004;14:155)
Note: thyroid uptake is also a nuclear medicine scan using I-131 or technetium (reference)
Note: T3 uptake in red blood cells is related to clinical depression (Physiol Res 2006;55:195)
Prohormone of T3 and T4 synthesized by thyroid follicular cells and cosecreted into serum with T3 and T4
Serum level is affected by thyroid mass, thyroid injury and TSH receptor stimulation
Tyrosine residues of thyroglobulin are iodinated with thyroid peroxidase to produce mono- and diiodotyrosyls, which combine to form T3 (mono+di) and T4 (di+di)
Thyroglobulin releases T3 and T4 into circulation via lysosomal degradation
Indications: used primarily to monitor recurrence of well differentiated thyroid cancer after total thyroidectomy or I-131 treatment (Endocr Regul 2006;40:53); also to diagnose thyroid dysgenesis in congenital hypothyroidism (J Med Genet 2005;42:379) or to distinguish subacute thyroiditis (elevated levels) from factitious thyrotoxicosis (undetectable levels despite elevated T3 and T4 levels)
Methodology: immunoassay
Monitoring recurrence of well differentiated thyroid carcinoma (J Clin Endocrinol Metab 2005;90:5047)
- give patient recombinant human TSH, and measure thyroglobulin 72 hours after last dose
- rise of serum thyroglobulin above 2 ng/ml (if had remnant ablation and receiving hormone suppressive therapy) correlates with presence of recurrent disease
- can confirm recurrent disease by whole body iodine scan
- well differentiated tumors typically have 10x rise in thyroglobuln
- poorly differentiated tumors may not concentrate iodide, may have blunted response to TSH stimulation
- can also monitor change of thyroglobulin level when receiving T4 treatment post-thyroidectomy
Limitations: 20% with thyroid cancer have antithyroglobulin antibodies which interfere with assay (falsely lower serum concentration by binding to thyroglobulin and making it “invisible” to the assay; should determine presence of antibodies when measure thyroglobulin level for tumor monitoring; if antibodies are present, PCR may be useful to measure thyroglobulin; also interference by heterophile antibodies (J Clin Endocrinol Metab 2003;88:3069)
Reference range: < 30 ng/ml (45 pmol/L) (chart)
High values: Graves’ disease, thyroiditis, nodular goiter, well differentiated thyroid carcinoma
Low levels: factitious thyrotoxicosis (surreptitious use of thyroid hormone-levels are undetectable although T3 or T4 are elevated)
See Stains chapter for immunohistochemistry using Thyroglobulin antibody
Indications: a marker of autoimmune thyroid disease (note: other thyroid antibodies are to TSH and thyroperoxidase); also ordered with serum thyroglobulin, since it may interfere with thyroglobulin assay
Modest correlation with presence of anti-thyroperoxidase antibodies (Autoimmunity 2006;39:497)
Methodology:
(a) immunoassays now most common
(b) quantitative passive hemagglutination: formerly most common, uses chromic chloride hemagglutination or tanned red blood cells, 80% with Hashimoto’s thyroiditis have titers > 1:1000; also present, at lower titers, in Graves’ disease (60%), thyroid carcinoma (30%), pernicious anemia, Sjogren’s syndrome, normals (3-18%)
(c) indirect immunofluorescence microscopy (uncommon): uses monkey thyroid gland tissue; patterns are floccular, dull colloid spaces but bright peripheral fluorescence or diffuse bright uniform colloid staining in ground-glass pattern (diagram-left); 5-8% are false positives with positive immunofluorescence attributed to CA2 (second colloid antigen) but are negative for anti-thyroglobulin or anti-thyroperoxidase by other methods
Concordance between automated methods needs improvement (Clin Chim Acta 2007;376:88)
Interpretation: present in patients with Hashimoto’s thyroiditis, other autoimmune disease, also women with no clinical disease; values < 4 .0 IU/ml are unlikely to interfere with thyroglobulin assay
Thyroid binding globulin (TBG)
Deficiency does not cause thyroid disease by itself
Increases/decreases cause increase/decrease in total T3 and T4; however free T3 and T4 are unchanged
Physiology: produced in liver; major carrier protein for T3 and T4; has highest affinity, but lowest concentration of the carrier proteins
Other thyroid carrier proteins are transthyretin and albumin, which have lower affinity but higher concentration
Indications: interpreting T3 and T4 levels that do not match clinical findings or other laboratory results (example - total T3 and T4 levels are very low, TSH is normal, free T3 and T4 are normal, patient is clinically euthyroid)
Methodology: chemiluminescence immunoassay
Reference range: 13-39 µg/dL (150-360 nmol/L) (chart)
High values: inherited abnormalities, drugs (clofibrate, estrogens, 5-FU, heroin/methadone), hepatitis, pregnancy, idiopathic
Low values: inherited abnormalities, drugs (androgens, glucocorticoids), liver failure, malnutrition, nephrotic syndrome, idiopathic
References: eMedicine (TBG deficiency), OMIM 314200
Theodor Kocher (Swiss surgeon) won Nobel Prize in 1909 for his work on thyroid physiology (award presentation speech)
Hypothalamus secretes thyrotropin releasing hormone (TRH), which acts on anterior pituitary to secrete thyroid stimulating hormone (TSH, thyrotropin), which increases thyroid follicular cell update of iodide and stimulates synthesis of T3 and T4; free T3 and free T4 act as negative feedback on hypothalamus and pituitary
Thyroid cells actively take up iodide, then iodinate tyrosine residues on thyroglobulin, then form mono- and di-iodotyrosine, which are coupled to form T3 and T4; thyroglobulin undergoes proteolysis to release free iodotyrosine, T4 and T3; iodotyrosines are deiodinated to reuse the iodide; T4 is deiodinated to T3</