Synonym

• T3
• T3 radioimmunoassay
• Triiodothyronine

Tubes

• Red, tiger, or green top tube
• 5-7 mL of venous blood

Additional information

• Discontinue any thyroid medication 1 month before the test
• Handle sample gently to prevent hemolysis
• Send sample to lab immediately

Info

• The triiodothyronine (T₃)radioimmunoassay measures the total amount of bound and free triiodothyronine hormone in the blood
• T₃ is synthesized and released from the thyroid gland in response to thyroid stimulating hormone (TSH) and indirectly in response to thyroid releasing hormone (TRH) similar to thyroxine (T₄)
• The rate of secretion of T₃ is normally regulated by a complex system of negative and positive feedback mechanisms
• In a euthyroid state, 20% of T₃ is synthesized from the thyroid gland and 80% is produced by enzymatic deiodination of T₄ in the tissues. T₃ once released into the blood binds with serum proteins such as thyroxine binding globulin (TBG), prealbumin, and albumin (99.7%). The only biologically active form is the free T₃ which makes up the remaining 0.3%.
• T₃ is 4-5 times more active metabolically than T₄ and binds less firmly to TBG, thereby having a shorter half life of 1 day

Clinical

• The clinical utility of the T₃ radioimmunoassay includes:
• Evaluation of thyroid function along with thyroid-stimulating hormone (TSH) and free T₄ assessment
• Aids in the diagnosis of T₃ thyrotoxicosis
• Aids in the diagnosis of hyperthyroidism and hypothyroidism when the TBG is normal
• As a neonatal screening test, for congenital hypothyroidism
• To monitor the therapeutic efficacy of antithyroid medication in hyperthyroidism or of thyroid replacement therapy in hypothyroidism
• To monitor thyroid disease progression
• To evaluate thyroid response to protein deficiency associated with severe illnesses
• T₃ toxicosis, presents as does hyperthyroidism, and is associated with increased T₃ levels, and normal or low T₄ levels. This condition occurs in 5% of patients diagnosed with hyperthyroidism; most commonly in areas of iodine deficiency, or in the elderly

• Hyperthyroidism presents as
• Anxious, restless, and fidgeting patient
• Fatigue
• Heat intolerance
• Frequent bowel movements, diarrhea
• Dermatologic
• Warm, moist and velvety
• Palmar erythema
• Hair is fine and silky
• Finger nails may show onycholysis or brown discoloration
• Neuromuscular
• Fine tremor of fingers, tongue
• Hyperkinesia
• Proximal muscle weakness
• Rapid speech
• Eye changes
• Widened palpebral fissures
• Infrequent blinking
• Chemosis
• Lid lag/Stare
• Proptosis (Exopthalmos)
• Periorbital edema
• Cardiovascular
• Palpitations
• Systolic hypertension
• Widened pulse pressure
• Atrial fibrillation
• Tachycardia
• Systolic murmur and loud S1, S2 heart sounds
• Cardiac hypertrophy
• Oligomenorrhea or amenorrhea
• Hypothyroid state may be clinically seen as:
• General
• Loss of energy, lethargy
• Cold intolerance
• Depression
• Weight gain
• Round puffy face or other facial edema (80%)
• Periorbital or eyelid edema (90%)
• Macroglossia (80%)
• Non-pitting ankle edema
• Neuropsychiatric
• Slow speech
• Hoarse voice
• Hypokinesia
• Generalized muscle weakness
• Delayed relaxation of knee and ankle jerk reflex
• Dermatologic
• Cold, dry, thick scaling skin affecting palms, soles, elbows and knees
• Skin may show orange-yellow discoloration
• Dry, coarse, brittle hair (75%)
• Loss of axillary, pubic, scalp hair
• Dry longitudinally ridged nails
• Cardiovascular
• Faint cardiac impulse
• Indistinct heart tones
• Cardiac enlargement
• Bradycardia
• Pericardial effusion
• Diastolic hypertension
• Gastrointestinal
• Constipation
• Ascites
• Menstrual disturbances, impaired fertility
• In severe cases, coma and respiratory failure may occur

Additional information

• Normal day to day variation is about 20% with higher level in winter and lower levels in summer
• T₃ levels are higher in men than in women
• T₃ is lower in neonates
• T₃ gradually decreases in those >60 years of age
• T₃ levels are of more diagnostic value in hyperthyroidism than hypothyroidism
• Factors interfering with test results include:
• Hemolysis causes false decrease in some assays
• Radioiodine, technetium used in scans may cause false positive results with radioimmunoassays
• Recent intake of medications affecting thyroid hormone levels
• Related laboratory tests include:
• Adrenocorticotropic hormone
• Antithyroglobulin and antithyroid peroxidase antibodies
• Cortisol
• Free T₃
• Free T₄
• T₄
• Thyroid-binding inhibitory immunoglobulin
• Thyroid-stimulating hormone
• Thyroid-stimulating immunoglobulin
• Thyroxine-binding globulin

Nl Result

Consult your laboratory for their normal ranges as these may vary somewhat from the ones listed below.

High Result

Conditions associated with elevated total T₃ levels, include:

• Hyperthyroidism
• Graves' disease
• Goiter
• T₃ thyrotoxicosis
• Conditions that increase TB
• Acute intermittent porphyria
• Biliary cirrhosis
• Familial dysalbuminemic hyperthyroxinemia
• Hepatitis
• HIV infection
• Newborn state
• Pancreatic and hepatic tumors
• Pregnancy
• Acute psychiatric illnesses
• Acute thyroiditis
• Early thyroid failure
• Hemoconcentration
• Hyperemesis gravidarum
• Iodine deficiency goiter
• Lymphoma
• Obesity
• Thyrotoxicosis factitia
• Drugs
• Amiodarone
• Amphetamines
• Benziodarone
• Clofibrate
• Cytomel (>25 µg daily dosage)
• Erythropoietin
• Estrogens
• Estropipate
• Ether
• Excess iodine
• Fenoprofen
• Fluorouracil
• Glucocorticoids
• Halofenate
• Heroin
• Insulin
• Iobenzamic acid
• Levarterenol
• Levodopa
• Levothyroxine
• Mestranol
• Methadone
• Opiates
• Oral contraceptives
• Perphenazine
• Phenothiazine
• Progestins
• Prostaglandins
• Raloxifene
• Ranitidine
• T₄ (>300 µg daily dosage)
• Tamoxifen
• Terbutaline
• Thyrotropin-releasing hormone

Low Result

Conditions associated with decreased total T₃ levels, include:

• Hypothyroidism
• Thyroid binding globulin deficiency states:
• Active Acromegaly (Rare)
• Chronic liver disease
• Genetic decrease of TBG (X linked)
• Carbohydrate deficient glycoprotein syndrome type 1 (CDG1)
• Nephrotic syndrome
• Obstructive jaundice
• Protein malnutrition
• Protein-losing states
• Severe systemic illness (except HIV/AIDS and acute intermittent porphyria)
• Surgical stress
• Acute stress
• Hypoproteinemia
• Nonthyroidal illness
• Panhypopituitarism
• Renal failure
• Smoking
• Strenuous exercise
• Drugs
• Acetylsalicylic acid
• Aminoglutethimide
• Aminosalicylic acid
• Anabolic steroids
• Androgens
• Anticonvulsants
• Barbiturates
• Beta blockers
• Carbamazepine
• Carbimazole
• Chlorpropamide
• Cholestyramine
• Cimetidine
• Clomiphene
• Clomipramine
• Cobalt
• Colestipol
• Corticotropin
• Cortisone
• Cotrimoxazole
• Cytostatic therapy
• Danazol
• Dehydroepiandrosterone
• Dexamethasone
• Diazepam
• Diazo dyes (evans blue)
• Dinitrophenol
• Ethionamide
• Fenclofenac
• Free fatty acids
• Furosemide
• Heparin
• Hydrocortisone
• Hydroxyphenylpyruvic acid
• Interferon alfa-2b
• Iobenzamic acid
• Iodides
• Iopanoic acid
• Iothiouracil
• Ipodate
• Iron
• Isotretinoin
• L-asparginase
• Liothyronine sodium
• Lithium
• Lovastatin
• Methimazole
• Methylthiouracil
• Mitotane
• Naproxen
• Neomycin
• Netilmicin
• Nicotinic acid
• Nitroprusside
• Norethindrone
• Penicillamine
• Penicillin
• Phenylacetic acid derivatives
• Phenylbutazone
• Phenytoin
• Potassium iodide
• Prednisone
• Propylthiouracil
• Radiographic agents
• Reserpine
• Rifampin
• Salsalate
• Somatostatin
• Stanozolol
• Sulfonylureas
• Tetrachlorothyronine
• Theophylline
• Tolbutamide
• Triiodothyronine (T₃)
• Trimethoprim-sulfamethoxazole
• Tyropanoic acid
• Valproic acid

References

• ARUP's Laboratories®. Triiodothyronine, Total (Total T3). [Homepage on the Internet] ©2007. Last accessed on April 2, 2007. Available at URL: http://www.aruplab.com/guides/ug/tests/0070474.jsp
• eMedicine from WebMD®. Hypothyroidism. [Homepage on the Internet] ©1996-2006. Last updated on September 23, 2006. Last accessed on April 2, 2007. Available at URL: http://www.emedicine.com/MED/topic1145.htm
• Konrady A. [T3-thyrotoxicosis: incidence, significance and correlation with iodine intake] [Article in Hungarian]. Orv Hetil. 2000 Feb 13;141(7):337-40.
• LabTestsOnline®. T3. [Homepage on the Internet]©2001-2007. Last reviewed on June 4, 2004. Last accessed on April 2, 2007. Available at URL: http://www.labtestsonline.org.uk/understanding/analytes/t3/glance.html
• Reid JR et al. Hyperthyroidism: diagnosis and treatment. Am Fam Physician. 2005 Aug 15;72(4):623-30. Available at URL: http://www.aafp.org/afp/20050815/623.html
• Pinelli M et al. Relationship between low T3 syndrome and NT-proBNP levels in non-cardiac patients. Acta Cardiol. 2007 Feb;62(1):19-24.
• Woeber KA et al. Triiodothyronine production in Graves' hyperthyroidism. Thyroid. 2006 Jul;16(7):687-90.