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thyroid function test

A collective term used for laboratory tests to check the function of the thyroid. Indications: - assessment of hypothyroidism - assessment of hyperthyroidism - monitoring thyroid replacement therapy Contraindications: - use in hospitalized patients in the absence of strong clinical suspicion of thyroid dysfunction [1] Physiology: - thyroid function is controlled by 2 main processes - pituitary secretion of TSH - peripheral conversion of T4 into T3 - T4 is a prohormone, T3 confers most of the physiological action of thyroid hormone Pathology: - non-thyroid illness is commonly associated with changes in thyroid function test; no treatment necessary - see specific thyroid disease - higher serum TSH & lower serum free T4 within the reference interval are associated with longer life expectancy (up to 3.5 years) [5] Laboratory: - serum TSH sensitive initial test for thyroid disorders [1] Drug interactions: 1) inhibition of TSH synthesis or release a) glucocorticoids b) dopamine c) dobutamine d) octreotide e) bexarotene 2) decreased thyroid hormone synthesis or release a) iodine b) lithium carbonate c) methimazole d) potassium perchlorate e) propylthiouracil 3) decreased conversion of T4 to T3 a) amiodarone (transient effect, < 3 months, in 85%) [1] b) corticosteroids c) ipodate, iopanoic acid d) propranolol (high dose) e) propylthiouracil 4) decreased thyroxine-binding globulin* a) androgens b) L-asparaginase c) glucocorticoids d) niacin 5) increased thyroxine-binding globulin* a) estrogen b) 5-fluorouracil c) heroin d) methadone e) raloxifene f) tamoxifen 6) enhanced metabolic clearance of T4# a) carbamazepine b) phenobarbital c) phenytoin d) rifampin e) sertraline 7) displacement of T4 from binding proteins a) aspirin b) carbamazepine c) furosemide d) heparin e) phenytoin f) salsalate 8) drugs inhibiting absorption or enterohepatic circulation$ of thyroxine (T4) a) aluminum hydroxide b) calcium c) cholestyramine, colestipol d) iron e) proton pump inhibitors f) psyllium g) raloxifene h) soybean oil i) sucralfate 9) drugs causing thyroiditis a) amiodarone b) interferon-alpha (autoimmune thyroiditis) c) interleukin-2 d) lithium carbonate e) tyrosine kinase inhibitors [1] 10) spurious laboratory results [1] a) amiodarone (high free T4, low serum T3, normal serum TSH) b) biotin > 5-10 mg/day (high free T4, low serum TSH) c) carbamazepine, oxcarbazepine, phenytoin - (low free T4, high serum T3, normal serum TSH) d) enoxaparin, heparin (high free T4, low serum T3, normal serum TSH) e) salsalate (low free T4, low serum T3, normal serum TSH) * affects total serum T4 & serum T3, but not free T4 or free T3 [1] # higher dose of thyroxine may be needed to normalize serum TSH in patients with hypothyroidism [1] $ separate from thyroxine dosing by several hours [1]

Related

thyroid thyroid disease

Specific

anti-thyroid peroxidase (microsomal) antibody free thyroxine index (fT4I) pentagastrin stimulation test (calcitonin-stimulation test) radioactive iodine uptake (RAIU) test T3 resin uptake T3 total in serum/plasma thyroglobulin antibody in serum thyroglobulin in serum thyroid-stimulating hormone (TSH) in serum; thyrotropin in serum thyroxine [T4] (free) in serum thyroxine [T4] (total) in serum TRH stimulation test triiodothyronine [T3] (free) in serum

General

clinical chemistry test

References

  1. Medical Knowledge Self Assessment Program (MKSAP) 15, 16, 17, 18, 19. American College of Physicians, Philadelphia 2009, 2012, 2015, 2018, 2022.
  2. Wikipedia: Thyroid function tests http://en.wikipedia.org/wiki/Thyroid_function_test
  3. Tabatabaie V, Surks MI. The aging thyroid. Curr Opin Endocrinol Diabetes Obes. 2013 Review. PMID: 23974775
  4. Demers LM, Spencer CA. Laboratory medicine practice guidelines: laboratory support for the diagnosis and monitoring of thyroid disease. Clin Endocrinol (Oxf). 2003 Feb;58(2):138-40. PMID: 12580927
  5. Bano A, Dhana K, Chaker L et al Association of Thyroid Function With Life Expectancy With and Without Cardiovascular Disease. The Rotterdam Study. JAMA Intern Med. Published online September 18, 2017 PMID: 28975207 http://jamanetwork.com/journals/jamainternalmedicine/article-abstract/2653451
  6. Burch HR Drug Effects on the Thyroid. N Engl J Med 2019; 381:749-761. Aug 22. PMID: 31433922 https://www.nejm.org/doi/full/10.1056/NEJMra1901214