II. Indication

  1. Evaluation of Thyroid Function
  2. Combined with Thyroid Stimulating Hormone (TSH)
    1. Together most accurate assessment of Thyroid status

III. Mechanism: Measurement

  1. Radioimmunoassay measures bound serum Thyroxine (T4)
  2. Free Thyroxine (Free T4) measures only active T4
    1. Not affected by Thyroid Binding Globulin (TBG)
    2. Free Thyroxine (Free T4) represents 0.03% of total T4

IV. Interpretation: Normal

  1. Thyroxine (T4): 4-12 ug/dl
  2. Free Thyroxine (Free T4): 0.7 - 1.9 ng/dl
  3. Pregnancy (Free T4)
    1. First trimester: 0.8 to 1.2 ng/dl
    2. Second trimester: 0.6 to 1.0 ng/dl
    3. Third timester: 0.5 to 0.8 ng/dl
    4. Abbassi (2010) Obstet Gynecol 114(6): 1326-31 [PubMed] (or open in [QxMD Read])

V. Causes: Increased Thyroxine (or Free T4)

  1. Most common
    1. Grave's Disease
    2. Toxic Multinodular Goiter
    3. Toxic adenoma
    4. Iatrogenic/factitious
    5. Transient Hyperthyroidism
      1. Subacute Thyroiditis,
      2. Hashimoto's Thyroiditis
  2. Uncommon to Rare
    1. Hyperthyroid secondary to pituitary disease
    2. Thyroid Cancer
    3. Amiodarone

VI. Causes: Decreased Thyroxine or Free T4

  1. Primary Hypothyroidism
    1. Hashimoto's Thyroiditis
    2. Idiopathic Myxedema
    3. Previous treatment of Hyperthyroidism
    4. Subacute Thyroiditis
    5. Prior neck radiotherapy
    6. Iodine deficiency or excess
    7. Congenital
    8. Medications
      1. Lithium
      2. PAS
      3. Sulfonamides
      4. Phenylbutazone
      5. Amiodarone
      6. Thiourea
  2. Secondary Hypothyroidism
    1. Pituitary dysfunction
  3. Tertiary Hypothyroidism
    1. Hypothalamic disease
  4. Very rare
    1. Tissue resistance to Thyroid hormone

Images: Related links to external sites (from Google)

Ontology: Thyroxine (C0040165)

Definition (CHV) thyroid hormone
Definition (CHV) thyroid hormone
Definition (CHV) thyroid hormone
Definition (CHV) thyroid hormone
Definition (CHV) thyroid hormone
Definition (CHV) thyroid hormone
Definition (NCI) A hormone that is made by the thyroid gland and contains iodine. T4 increases the rate of chemical reactions in cells and helps control growth and development. T4 can also be made in the laboratory and is used to treat thyroid disorders.
Definition (MSH) The major hormone derived from the thyroid gland. Thyroxine is synthesized via the iodination of tyrosines (MONOIODOTYROSINE) and the coupling of iodotyrosines (DIIODOTYROSINE) in the THYROGLOBULIN. Thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. Thyroxine is peripherally deiodinated to form TRIIODOTHYRONINE which exerts a broad spectrum of stimulatory effects on cell metabolism.
Definition (CSP) tetraiodothyronine (T4); major hormone elaborated by the thyroid gland follicular cells; increases the rate of cell metabolism, essential for central nervous system maturation, and regulates a number of other functions.
Definition (NCI) A hormone synthesized and secreted by the thyroid gland containing four iodine atoms and is converted to triiodothyronine (T3) in the body, influencing metabolism and organ function.
Definition (PDQ) A hormone synthesized and secreted by the thyroid gland containing four iodine atoms and is converted to triiodothyronine (T3) in the body, influencing metabolism and organ function. Check for "http://www.cancer.gov/Search/ClinicalTrialsLink.aspx?id=528273&idtype=1" active clinical trials or "http://www.cancer.gov/Search/ClinicalTrialsLink.aspx?id=528273&idtype=1&closed=1" closed clinical trials using this agent. ("http://nciterms.nci.nih.gov:80/NCIBrowser/ConceptReport.jsp?dictionary=NCI_Thesaurus&code=C2302" NCI Thesaurus)
Concepts Amino Acid, Peptide, or Protein (T116) , Pharmacologic Substance(T121) , Hormone(T125)
MSH D013974
SnomedCT 73187006, 38076006
English 3,5,3',5'-Tetraiodothyronine, L Thyroxine, L-3,5,3',5'-Tetraiodothyronine, L-Thyroxine, Levothyroxine, Thyroxin, Thyroxine, O-(4-Hydroxy-3,5-diiodophenyl) 3,5-diiodo-L-tyrosine, O-(4-Hydroxy-3,5-diiodophenyl)-3,5-diiodotyrosine, Tyrosine, O-(4-hydroxy-3,5-diiodophenyl)-3,5-diiodo-, levothyroxine, T4, T4 preparation, T<sub>4</sub> preparation, T<sub>4</sub>, thyroxine, T AAB THYROID HORMONE 004, THROXINE L, Thyroxine [Chemical/Ingredient], l-thyroxine, thyroxine (T4), l thyroxine, LEVOTHYROXINE, thyroxin, L-3,5,5'-tetraiodothyronine, T4 Thyroid Hormone, Thyroid Hormone, T4, 3,5,3'5' Tetraiodothyronine, Tetraiodothyronine preparation, Thyroxine preparation, Thyroxine product, T4 - Thyroxine, Total thyroxine, TT4 - Total thyroxine, T>4< preparation, T>4<, Tetraiodothyronine preparation (product), Thyroxine (substance), Tetraiodothyronine preparation (substance)
Portuguese L-Tireoxina, Levotireoxina, Tetraiodotironina, Tireotoxina, Levotiroxina, Tirotoxina, Tireoxina, T4, Tiroxina, Hormônio T4 da Tireóide, Hormônio Tireóideo T4
Swedish Tyroxin
Czech thyroxin, hormon T4, tyroxin
Finnish Tyroksiini
French T4 (Hormone thyroïdienne), Lévothyroxine, L-Thyroxine, 3,5,3',5'-Tétraiodothyronine, Hormone thyroïdienne T4, Tétra-iodothyronine, Tétraiodothyronine, Thyroxine
Russian L-3,5,3',5'-TETRAIODTIRONIN, L-TIROKSIN, TIROKSIN, LEVOTIROKSIN, L-3,5,3',5'-ТЕТРАИОДТИРОНИН, L-ТИРОКСИН, ЛЕВОТИРОКСИН, ТИРОКСИН
Japanese サイロキシン, テトラヨードチロニン, レボチロキシンナトリウム水和物, L-チロキシン, チラージンS, レボチロキシンナトリウム, チラーヂンS, フリーT4, チロキシン, レボサイロキシン, レボチロキシン, L-3,5,3',5'-テトラヨードチロニン
Spanish T4, preparado con tetrayodotironina (producto), tetrayodotironina, preparado de tiroxina, tetrayodotironina (producto), preparado con tetrayodotironina, 3, 5, 3'5' tetrayodotironina, preparado de T>4, preparado de tetrayodotironina (producto), preparado de tetrayodotironina (sustancia), preparado de tetrayodotironina, tiroxina (sustancia), tiroxina, Tiroxina, Hormona Tiroidea T4, Levotiroxina, Tiroxina-L
German T AAB SCHILDDRÜSENHORMON 004, THYROXIN A L, L-Thyroxin, Laevothyroxin, Levothyroxin, Thyroxin, T4-Schilddrüsenhormon
Croatian TIROKSIN
Polish L-tyroksyna, Tyroksyna, Sól sodowa lewotyroksyny, Lewotyroksyna
Italian Tiroxina

Ontology: T4 free measurement (C0202225)

Definition (NCI) The determination of the amount of free thyroxine present in a sample.
Definition (NCI) A measurement of the free thyroxine in a biological specimen.
Concepts Laboratory Procedure (T059)
SnomedCT 143689003, 5113004
CPT 84439
English T4 free measurement, Thyroxine; free, THYROXINE; FREE, Free thyroxine level (procedure), Free T4, Free Thyroxine Measurement, T4FR, Thyroxine, Free, unbound T4 level, free thyroxine measurement (lab test), free thyroxine measurement, free T4, free thyroxine (T4) level test, Thyroxine free, FT4 measurement, Free thyroxine measurement, Unbound T4 measurement, Free T4 measurement, FT4 - Free thyroxine level, T4 free measurement (procedure), Free thyroxine level, ASSAY OF FREE THYROXINE, Assay of free thyroxine, THYROXINE FR
Italian Tiroxina libera, T4 libero
Dutch vrij T4, thyroxine vrij
French T4 libre, Thyroxine libre
German freies T4, Thyroxin frei
Portuguese T4 livre, Tiroxina livre
Spanish T4 libre, Free thyroxine level, libre, medición de FT4, medición de T4 libre (procedimiento), medición de T4 libre, medición de T4 no ligado, medición de tiroxina libre, medición libre de T4, Tiroxina libre
Japanese 遊離サイロキシン, 遊離T4, ユウリサイロキシン, ユウリT4
Czech T4 volný, Tyroxin volný
Hungarian Szabad tiroxin, Szabad T4