The thyroid is unique among the endocrine glands because it has a large store of hormone and a slow rate of normal turnover. Stimulation of the thyroid gland by TSH, which is produced by the anterior pituitary gland, causes the release and distribution of stored thyroid hormones. TSH stimulates secretion of T4 and T3. TSH secretion is physiologically regulated by T3 and T4 (feedback inhibition) and is stimulated by TRH from the hypothalamus. TSH is the single most sensitive test for primary hypothyroidism. If there is clear evidence of hypothyroidism and the TSH is not elevated, then an implication of possible hypopituitarism exists.
This measurement is used in the diagnosis of primary hypothyroidism when there is thyroid gland failure owing to intrinsic disease, and it is used to differentiate primary from secondary hypothyroidism by determining the actual circulatory level of TSH. TSH levels are high in primary hypothyroidism. Low TSH levels occur in hyperthyroidism.
TSH measurements with sufficient sensitivity to distinguish low levels from normal levels have become the preferred test for hyperthyroidism. The third-generation TSH test is useful for diagnosing sick patients with euthyroidism and in differentiating mild hyperthyroidism from Graves disease. With the new, sensitive assays, a TRH stimulation test is no longer necessary.
Adults: 0.45–4.5 μIU/mL or 0.45–4.5 mIU/L
Children (6–10 years): 0.66–4.14 μIU/mL or 0.66–4.14 mIU/L
Adolescents (11–19 years): 0.53–3.59 μIU/mL or 0.53–3.59 mIU/L
Neonates (1–3 days): 5.17–14.6 μU/mL or 5.17–14.6 mU/L
Clinical Alert
Critical ValuesValues 0.1 mIU/L are an indication of primary hyperthyroidism or exogenous thyrotoxicosis. Risk exists for atrial fibrillation at TSH levels 0.1 mIU/L (major risk factor for stroke).
Obtain a 5-mL venous blood sample (red-topped tube). Serum is needed.
Observe standard precautions. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.
Increased TSH levels are seen in the following conditions:
Adults and neonates with primary hypothyroidism
Thyrotropin-producing tumor (e.g., ectopic TSH secretion from lung, breast tumors)
Hashimoto thyroiditis
Thyrotoxicosis due to pituitary tumor
TSH antibodies (rare)
Patients with hypothyroidism receiving insufficient thyroid replacement hormone or with thyroid hormone resistance
Decreased TSH levels are associated with the following conditions:
Primary hyperthyroidism
Secondary and tertiary hypothyroidism
Treated Graves disease
Euthyroid sick syndrome
Overreplacement of thyroid hormone in treatment of hypothyroidism
Pretest Patient Care
Explain test purpose and procedure.
Follow guidelines in Chapter 1 for safe, effective, informed pretest care.
Posttest Patient Care
Have patient resume normal activities.
Review test results; report and record findings. Modify the nursing care plan as needed. Counsel as appropriate for hypothyroidism or hyperthyroidism.
Follow guidelines in Chapter 1 for safe, effective, informed posttest care.
Values are normally high in neonatal cord blood. There is hypersecretion of TSH in newborns up to 2–3 times normal. The TSH level approaches normal by the first week of life.
Values are suppressed during treatment with T4 and corticosteroids. see Appendix E for other drugs.
Values are abnormally increased with lithium, potassium iodide, amphetamine abuse, and iodine-containing drugs.
Radioisotopes administered within 1 week before test invalidate the result.
Values may be decreased in the first trimester of pregnancy.
Values are increased in older adult patients (older than 80 years); upper limit for these patients is 10 μIU/mL or 10 mIU/L.
Heterophilic antibodies may falsely increase or decrease test results.