Chemistry
Thyroid related
Thyroid stimulating hormone (TSH)

Author: Nat Pernick, M.D. (see Authors page)

Revised: 23 February 2016, last major update March 2011

Copyright: (c) 2002-2016, PathologyOutlines.com, Inc.

PubMed Search: Thyroid stimulating hormone [title]
Cite this page: Thyroid stimulating hormone (TSH). PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/chemistrytsh.html. Accessed December 2nd, 2016.
Definition / General
  • Also called thyrotropin
  • Promotes increased uptake of iodide into thyroid follicular cells by active transport, synthesis of thyroglobulin, formation of T3 and T4, release of T3 and T4 into circulation
  • Composed of alpha subunit (same sequence as LH, FSH, hCG) and beta unit (differs from other hormones)
Laboratory
Indications
  • Initial screening test for hyper- or hypothyroidism (Am J Clin Pathol 1996;105:11), unless:
    • Hypothalamic-pituitary-thyroid axis is not intact due to disease or medication
    • There is thyroid hormone resistance
    • There was recent therapy for hyper- or hypothyroidism
      • For these cases, measure free T4 and possibly also free T3 or total T4
  • Annual screening for patients receiving thyroid hormone replacement therapy (wait 6 weeks to retest due to altered dose)
  • Monitor patients with thyroid carcinoma and hormone suppressive therapy (target is 0.5 to 0.10 μU/mL if low risk and < 0.1 μU/mL if high risk
  • Screening of elderly (low yield for other populations) and pregnant women

  • First generation:
    • RIA
    • Not useful for detecting hyperthyroidism
  • Third generation:
    • Uses chemiluminescent label
    • Detects TSH of .005 mIU/L
    • American Thyroid Association recommends using assays that have a coefficient of variation of 20% or less for interassay precision
  • Fourth generation:
    • Not widely available yet
    • More sensitive than third generation, but not needed for most clinical applications
  • TSH within reference range excludes thyroid dysfunction, but may have normal function with abnormal TSH (low TSH in elderly, clinically euthyroid, normal T4)
Interpretation
  • <0.01 mIU/L:
    • Severe thyrotoxicosis (suggestive of Graves disease)
    • Also nonthyroidal illness plus dopamine or somatostatin
  • 0.01 to 0.1 mIU/L:
    • Mild or subclinical hyperthyroidism or acute nonthyroidal illness
    • Also hypothyroidism due to pituitary disorders, glucocorticoids
  • 0.5 to 2.0 mIU/L:
    • Target range for replacement thyroxine in primary hypothyroidism
  • 0.5 to 5.0 mIU/L:
    • Standard reference range
    • May include hyperthyroid patients with TSH producing tumors or pituitary resistance to thyroid hormone
    • Some recommend lowering upper limit to 2.5 mIU/L (J Clin Endocrinol Metab 2005;90:5483)
  • >5.0 mIU/L:
See also
Nonthyroidal illness
  • Low TSH acutely, high TSH with resolution

Pregnancy
  • Low TSH in first trimester due to TSH-like effects of hCG

Third trimester amniotic fluid (Bayer ADVIA Centaur)
  • Reference range is 0.04 to .51 mIU/mL, median 0.10 mIU/mL (Am J Clin Pathol 2007;128:158)
  • Notes:
    • Henry does not believe reference range should be age adjusted (excluding children), because mild abnormalities may be associated with increased mortality
    • Within the reference range, increasing TSH concentrations are associated with increasing cardiovascular risk parameters (Thyroid 2007;17:243)
    • Recommended to retest patients with TSH of 2.5 to 5.0 every 1-2 years (J Clin Endocrinol Metab 2005;90:5489)
Additional References