Thyroid-stimulating hormone (TSH) is produced by the basophil cells of the adenohypophysis in response to stimulation by its hypothalamic releasing factor, thyrotropin-releasing hormone (TRH). TRH responds to decreased circulating levels of thyroid hormones, as well as to intense cold, psychological tension, and increased metabolic need, and it stimulates the adenohypophysis to secrete TSH. TSH accelerates all aspects of hormone production by the thyroid gland and enhances hPRL release. Measuring TSH provides useful information about both hypophyseal and thyroid gland function.

Hypersecretion of TSH by the adenohypophysis (e.g., because of TSH-secreting pituitary tumors) causes hyperthyroidism as a result of excessive stimulation of the thyroid gland. Elevated TSH levels are also seen with prolonged emotional stress and are more common in colder climates. Primary hypothyroidism (i.e., hypothyroidism caused by disorders involving the thyroid gland itself) leads to elevated TSH levels because of normal feedback mechanisms. TSH levels are normally elevated at birth.

Note that increased TSH secretion is associated with excess secretion of exophthalmos-producing substance, which also originates in the adenohypophysis. This substance promotes water storage in the retro-orbital fat pads and causes the eyes to protrude, a common sign of hyperthyroidism. Exophthalmos sometimes persists after the hyperthyroidism is corrected and also may occur in persons with normal thyroid function.

TSH levels are normal in situations in which the functional ability of the thyroid gland is normal but the thyroid hormone levels are low, a phenomenon that is seen in clients with severe illnesses with protein deficiency (thyroid hormones are proteins) such as neoplastic disease, severe burns, trauma, liver disease, renal failure, and cardiovascular problems. Deficiency of thyroid hormone produces a hypometabolic state. Excess TSH production is not stimulated, however, because circulating thyroid levels are appropriate to the client's metabolic needs (i.e., the person is metabolically euthyroid). Treat-ment involves correcting the underlying causes. The apparent hypothyroidism is not treated, however, because such treatment could be devastating to a severely debilitated person.

TSH is measured by radioimmunoassay. Immunologic cross-reactivity occurs with glycoprotein hormones such as human chorionic gona-dotropin (hCG), follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

• Aspirin, adrenocorticosteroids, and heparin may produce decreased TSH levels.
• Lithium carbonate and potassium iodide may produce elevated TSH levels.
• Falsely increased levels may occur in hydatidiform mole, choriocarcinoma, embryonal carcinoma of the testes, pregnancy, and postmenopausal states characterized by high FSH and LH levels.⁴²

• Signs and symptoms of hypothyroidism, hyperthyroidism, or suspected pituitary or hypothalamic dysfunction, or hypothyroidism or hyperthyroidism combined with suspected pituitary or hypothalamic dysfunction:
  • Elevated levels are seen with primary hypothyroidism.
  • Decreased or undetectable levels are associated with secondary hypothyroidism caused by pituitary or hypothalamic hypofunction.
  • Decreased levels are seen with primary hyperthyroidism.
  • Elevated levels may indicate secondary hyperthyroidism resulting from pituitary hyperactivity (e.g., caused by tumor).
• Differentiation of functional euthyroidism from true hypothyroidism in debilitated individuals, with the former indicated by normal levels

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving collection of a peripheral blood sample (see Appendix I).

• It is recommended that drugs known to alter TSH levels be withheld for 12 to 24 hours before the test, although this practice should be confirmed with the person ordering the study.

A venipuncture is performed and the sample is collected in a red-topped tube. The sample should be handled gently to avoid hemolysis and transported promptly to the laboratory.

• The test for TSH is used on newborns to screen for congenital hypothyroidism. It is performed by obtaining a sample of blood from a heelstick and saturating a spot on a special filter paper with the blood. A kit is available for this test; it contains a comparison chart to identify elevations.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Resume any medications withheld before the test.
• Complications and precautions: Note the relation of TSH to levels of other thyroid tests indicating hypothyroidism as opposed to other thyroid disorders.