EBM Consult

Lab Test: Thyroid Stimulating Hormone, TSH Level

    Lab Test
    • Thyroid Stimulating Hormone (TSH)
    • Measurement of thyroid stimulating hormone (TSH) in serum for the evaluation and management of thyroid dysfunction. 
    • Helps to differentiate primary hypothyroidism from secondary (pituitary) and tertiary (hypothalamus) hypothyroidism.
    Reference Range
    • Adults = 0.5 - 5.0 microunits/mL (0.5 - 5.0 milliunits/L)
    • Pregnancy:
      • 1st trimester = 0.3 - 4.5 microunits/mL (0.3 - 45 milliunits/L)
      • 2nd trimester = 0.5 - 4.6 microunits/mL (0.5 - 4.6 milliunits/L)
      • 3rd trimester = 0.8 - 5.2 microunits/mL (0.8 - 5.2 milliunits/L)
      • Newborn = 3 - 18 microunits/mL (3 -18 mU/L)
        • Cord = 3 - 12 microunits/mL (3 - 12 mU/L)
      Indications & Uses
      • Suspected and known atrial fibrillation (AF):
        • Subnormal TSH is consistent with possible hyperthyroidism. 
        • Subclinical hyperthyroidism is a reversible cause of AF in elderly patients and is relatively common. A low serum TSH may indicate a 3-fold increase in developing AF over the next 10 years. Tests of thyroid function are indicated for first episodes of AF, episodes where ventricular rate is difficult to control, in elderly individuals presenting with AF, and when AF recurs unexpectedly after cardioversion. 
      • Suspected and known hypothyroidism:
        • a TSH level > 20 milli-International Units/L in association with a low free thyroxine (T4) confirms the diagnosis of hypothyroidism.  In some cases, levels may be as high as 200 milli-International Units/L.  Lower elevations (< 20 milli-International Units/L), in conjunction with a normal thyroid hormone level, may indicate subclinical hypothyroidism.  In secondary hypothyroidism, TSH levels may be normal, low, or high. 
        • In thyroid replacement therapy for hypothyroidism, the usual therapeutic goal is to achieve a TSH level in the mid-normal range (1 to 2 milli-International Units/L).
        • In thyroid replacement for subclinical hypothyroidism, the usual target TSH level is 0.03 to 3 milli-International Units/L).
      • Suspected hyperthyroidism:
        • a TSH level < 0.1 milli-International Units/L strongly suggests the diagnosis of hyperthyroidism, particularly when accompanied by a high free T4 or high free T3 level. 
        • If a 3rd generation assay is not available, T4 and T3 levels should be measured in addition to TSH to arrive at an accurate diagnosis.
        • In patients with subclinical hyperthyroidism, the TSH level may be slightly higher ranging from <0.1 milliInternational Units/L to the lower limit of the normal range.
        • In hyperthyroid patients who undergo treatment with radioactive iodine, an elevated TSH in conjunction with a normal T4 indicates a 5% to 15% risk of progression to hypothyroidism at 1 year.
      • Gout:
        • In patients with urate crystals present in synovial fluid indicating gout, a TSH level aids in assessing thyroid function. 
        • Hypothyroidism may be associated with gout.
      Clinical Application
      • Pituitary TSH secretion is stimulated by hypothalamic thyroid-releasing hormone (TRH). 
      • Low levels of triodothyronine (T3) and thyroxine (T4) are the underlying stimuli for TRH and TSH. 
      • Therefore a compensatory elevation of TRH and TSH occurs in patients with primary hypothyroid states, such as surgical or radioactive thyroid ablation; in patients with burned-out thyroiditis, thyroid agenesis, idiopathic hypothyroidism, or congenital cretinism; or in patients taking antithyroid medications. 
      • This test may be used to detect primary hypothyroidism in newborns with low screening T4 levels.  TSH is the single most reliable test for detecting thyroid disorders. 
      • Increased TSH levels may indicate:
        • Primary hypothyroidism (thyroid dysfunction), thyroiditis, thyroid agenesis, congenital cretinism, large doses of iodine, radioactive iodine injection, surgical ablation of thyroid, severe and chronic illnesses, or pituitary TSH-secreting tumor. 
      • Decreased TSH levels may indicate:
        • Secondary hypothyroidism (pituitary or hypothalamus dysfunction), hyperthyroidism, suppressive doses of thyroid medication, or factitious hyperthyroidism Results increased in:  >Hospitalized patients, acute psychiatric illness, adrenal insufficiency Results decreased in: Hospitalized patients, blacks, first trimester pregnancy
      Related Tests
      • Anemia panel
      • General health panel
      • Thyroid panel
      • Transplant panel
      • Thyroid-stimulating immunoglobulins
        • LATS and other thyroid-stimulating immunoglobulins are used to support the diagnosis of Graves disease, especially when the diagnosis is complex.
      • Thyrotropin-releasing hormone (TRH) stimulation test
        • Assists in the evaluation of patients with hyperthyroidism and hypothyroidism. 
      • Thyroxine-binding globulin (TGB)
        • Used in the evaluation of patients who abnormal total T4 (thyroxine) and T3 levels. 
      • Thyroxine (T4), total
        • sed to diagnose thyroid function and to monitor replacement and suppressive therapy.
      • Thyroxine index, free
        • Used to evaluate thyroid function.  It corrects for changes in thyroid hormone-binding serum proteins that can affect total T4 levels. 
      • Thyroxine (T4), free
        • Used to evaluate thyroid function in patients who may have protein abnormalities that could affect total T4 levels. 
      • Antithyroglobulin antibody
        • Used primarily in the differential diagnosis of thyroid diseases, such as Hashimoto thyroiditis and chronic lymphocytic thyroiditis (in children)
      Drug-Lab Interactions
      • Recent radioisotope administration may affect test results.
      • Severe illness may cause decreased TSH levels. 
      • There is a diurnal variation in TSH levels.  Basal levels occur around 10 AM and highest levels (about two to three times basal levels) occur around 10 PM.
      • Drugs that may cause increased levels include:  antithyroid medications, lithium, potassium iodide, and TSH injection.
      • Drugs that may cause decreased levels include:  aspirin, heparin, nonsteroidal antiarthritics dopamine, steroids, and T3.
      Test Tube Needed
      • Red marbled tube
      • Collect a venous blood sample. 
      • Use a heel stick to obtain blood from newborns.
      • Apply pressure or a pressure dressing to the venipuncture site and assess the site for bleeding.
      Storage and Handling
      • Store at 4°C for up to 7 days or freeze for up to 1 month.
      What To Tell Patient Before & After
      • Explain the procedure to the patient.
      • Tell the patient that no food or drink restrictions are necessary.
      • Baskin HJ et al. Endocr Pract 2002;8(6):457-69.
      • Ladenson PW et al. Arch Intern Med 2000;160:1573-5.
      • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
      • Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.

    MESH Terms & Keywords

    • Thyroid, TSH, Thyroid Stimulating Hormone, Thyroid Gland, TSH Lab Test