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commonly used endocrine tests are discussed in this chapter. Clinicians should refer to their particular institution's laboratory values (generally located in the back of the clinical record) for normal

ranges of hormone or hormone-related substances referenced in

their setting.

Clinical Tip

An imbalance of hormone levels may affect the patient's

tolerance to activity. Familiarity with the endocrine tests

and values can help the clinician gauge the intended treatment parameters (i.e., type, duration, and intensity) for the

next session(s).

ENDOCRINE SY5fEM

653

Thyroid Gland

Function

The thyroid gland secretes three hormones: thyroxine (T4), triiodothyronine (T3), and calcitionin, with T4 and T3 commonly referred to as rhe thyroid hormones. Thyroxine and triiodothyronine require

the presence of adequate amounts of iodine to be properly synthesized. Therefore, dietary deficiencies of iodine can hinder thyroid hormone production. The production and secretion of thyroid hormones are regulated by thyrotropin (also called thyroid stimulating hormolle [TSHJ), which is secreted from the anterior pituitary

gland. TSH levels are directly influenced by T4 levels through a negative feedback loop. Thyrotropin is further regulated by thyrotropin releasing hormone, which is secreted from the hypothalamus.'-6

Table 11-1 summarizes the target sites and actions of the thyroid

gland hormones.

Thyroid Tests

Thyroid hormones T4 and T3 circulate throughout the bloodstream

bound to proteins (approximately 99%) or unbound, in which case

Table 11-1. Target Sires and Acrions of Thyroid Gland Hormones

Hormone(s)

Target Site(s)

Actions

Thyroxine (T4)

Systemic

Increases merabolic rare. stimulates

and triiodothygrowth, and development of al!

ronine (T3)

cel!s, particularly of the nervous system, and enhances the effecrs of catechola mines

Thyrocaicicollin

Bone

Inhibits bone resorption

Lowers blood levels of calcium

Sources: Data from SF Fuller. Anatomy and Physiology of the Endocrine System. In

eM Hudak, 8M Gallo (eds), Critical Care NurSing: A Holistic Approach (6th ed). Philadelphia: Lippincott, 1994;75; M Harrog (cd). Endocrinology. Oxford. UK: Blackwell Scientific, 1987;23; and JV Corbect. Laboratory Tests and Diagnostic Procedures with

Nurslllg Diagnoses (5th cd). Upper Saddle River, NJ; Prentice Hall Health, 2000;409.