Type of test Blood
Normal findings
Adult: 2-10 μU/mL or 2-10 mU/L (SI units)
Newborn: 3-18 μU/mL or 3-18 mU/L
Cord: 3-12 μU/mL or 3-12 mU/L
(Values vary among laboratories.)
Test explanation and related physiology
The TSH concentration aids in differentiating primary from secondary hypothyroidism. Pituitary TSH secretion is stimulated by hypothalamic thyroid-releasing hormone (TRH). Low levels of triiodothyronine and thyroxine (T3 and 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; patients with burned-out thyroiditis, thyroid agenesis, idiopathic hypothyroidism, or congenital hypothyroidism; and patients taking antithyroid medications.
In secondary hypothyroidism, the function of the hypothalamus or pituitary gland is faulty because of tumor, trauma, or infarction. Thus TRH and TSH cannot be secreted, and plasma levels of these hormones are near 0 despite low T3 and T4 levels.
The TRH stimulation test is sometimes used to stimulate low levels of TSH to identify primary from secondary hypothyroidism in cases in which TSH is low. However, this test is not commonly used because extremely low levels of TSH can be identified now with the use of immunoassays.
The TSH test is used as well to monitor exogenous thyroid replacement. The goal of thyroid replacement therapy is to provide an adequate amount of thyroid medication so that TSH secretion is in the low normal range, indicating a euthyroid state. This test is also done to detect primary hypothyroidism in newborns with low screening T4 levels. TSH and T4 levels are frequently measured to differentiate pituitary from thyroid dysfunction. A decreased T4 with a normal or elevated TSH level can indicate a thyroid disorder. A decreased T4 with a decreased TSH level can indicate a pituitary disorder.
Interfering factors
• Severe illness may cause decreased TSH levels.
* Drugs that may cause increased levels include antithyroid medications, lithium, potassium iodide, and TSH injection.
* Drugs that may cause decreased levels include aspirin, dopamine, heparin, steroids, and T3 .
Procedure and patient care
• See inside front cover for Routine Blood Testing.
• Fasting: no
• Blood tube commonly used: red
• Use a heel stick to obtain blood from newborns.
Abnormal findings
Increased levels
- Primary hypothyroidism (thyroid dysfunction)
- Thyroiditis
- Thyroid agenesis
- Congenital hypothyroidism
- Large doses of iodine
- Severe and chronic illnesses
- Pituitary TSH-secreting tumor
Decreased levels
- Secondary hypothyroidism (pituitary dysfunction)
- Hyperthyroidism
- Pituitary hypofunction
