Clinical Controversies in Screening Women for Thyroid Disorders During Pregnancy

Frances A. Wier, CNM, MS; Cindy L. Farley, CNM, PhD

Disclosures

J Midwifery Womens Health. 2006;51(3):152-158. 

In This Article

Thyroid Physiology

The thyroid gland produces the hormones L-thyroxine (T4) and L-triiodothyronine (T3), which regulate metabolic body processes, cellular respiration, total energy expenditure, growth and maturation of tissues, and turnover of hormones, substrates, and vitamins.[8,9] The gland is composed of a uniform cluster of follicles enclosed by a thin, fibrous capsule surrounded by capillaries. The follicles are the structural, functional, and secretory units of the thyroid gland.

Thyroperoxidase (TPO) is one of the primary enzymes produced in the thyroid. It is synthesized within the endoplasmic reticulum of the thyrocyte and oxidizes iodine,[10] thereby facilitating the formation of T3 and T4. Iodine is a critical component of thyroid hormones and composes 65% of T4 weight and 58% of T3 weight. T3 is the active hormone (3 times the metabolic potency of T4), and T4 is the prohormone, broken down in the tissues to form T3 as needed.

Release of the hormones into the bloodstream involves the negative feedback system of the hypothalamic-pituitary-thyroid axis.[9] A low metabolic rate or a decrease in serum T3 and/or T4 levels signals the hypothalamus to secrete thyrotropin releasing hormone (TRH), which travels to the anterior pituitary gland and stimulates secretion of thyroid-stimulating hormone (TSH). An elevated T3 serum level inhibits release of TRH and TSH.[10] Expert opinion on the normal serum TSH level varies from the AACE's 0.3 to 3.0 mIU/L to the National Academy of Clinical Biochemistry's 0.4 to 2.5 mIU/L.[1,11,12] However, most agree that the upper level of normal should not be > 4.12 mIU/L.

TSH, in turn, stimulates the thyroid gland to manufacture and release stored T3 and T4 until the metabolic rate is normalized.[9] Thyroxine-binding globulin (TBG) is the primary protein that binds to T3 and T4 in the plasma. Unbound or free hormones are available to the tissue. The normal range of serum TSH concentration in the euthyroid population was found to be 0.4 to 2.5 mIU/L by the National Academy of Clinical Biochemistry, and supports the redefined TSH reference range (0.3-3.0 mIU/L) recommended by the AACE.[1] A high serum iodine concentration will also suppress the release of thyroid hormones.

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