Hypothyroidism

Published on 10/03/2015 by admin

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Chapter 21 HYPOTHYROIDISM

Hypothyroidism usually results from decreased thyroid hormone production and secretion by the thyroid gland. Hypothyroidism can be either primary or central. Central hypothyroidism involves either the pituitary gland (secondary hypothyroidism) or the hypothalamus (tertiary hypothyroidism). In the United States, the most common cause of hypothyroidism is chronic autoimmune thyroiditis.

Hypothyroidism is far more common in women and in the elderly than in other populations. In fact, about 2% to 3% of older women have hypothyroidism. Other risk factors include the presence of thyroid peroxidase antibodies and a high normal level of TSH.

Untreated hypothyroidism can result in decreased cardiac output, memory loss, infertility, and sleep apnea. The American Academy of Family Physicians recommends screening for hypothyroidism in patients 60 years of age or older and in patients with symptoms of hypothyroidism, a family history of thyroid disease, a history of autoimmune disease, or type 1 diabetes.

Myxedema coma refers to severe complications of hypothyroidism, involving hypothermia and stupor or coma. Myxedema coma can be precipitated by mild illnesses, exposure to cold, myocardial infarction, and medications that affect the central nervous system.

During pregnancy, there is a greater requirement of thyroid hormone because of increased maternal use, as well as transportation of thyroid hormone across the placenta. Some women who were euthyroid before pregnancy become hypothyroid during pregnancy. It is very important that women be treated adequately for hypothyroidism during pregnancy to avoid complications such as miscarriage, preeclampsia, preterm labor, and postpartum hemorrhage.

Medications That Can Cause Hypothyroidism

Propylthiouracil

Causes of Hypothyroidism

Primary Hypothyroidism

Secondary/Tertiary Hypothyroidism (Central Hypothyroidism)

• Chronic lymphocytic hypophysitis

• Congenital abnormalities (defects in thyrotropin-releasing hormone, TSH, or both)

• Infiltrative disorders

• Other brain tumors (nonpituitary)

• Pituitary tumors, metastasis, hemorrhage, necrosis, and aneurysms

Suggested Work-Up

TSH measurement	Levels are high in primary hypothyroidism and subclinical hypothyroidism; levels are low, normal, or minimally elevated in central hypothyroidism
Free thyroxine (T4) measurement	Levels are low in primary and central hypothyroidism, normal in subclinical hypothyroidism, and high in peripheral thyroid hormone resistance

Additional Work-Up

Total triiodothyronine (T3) measurement	Less useful than T4 measurement because T3 level may be normal in patients with hypothyroidism
Measurement of thyroid peroxidase autoantibodies	These are often detectable in patients with Hashimoto thyroiditis (measurement is useful in patients with subclinical hypothyroidism)
Measurement of thyroglobulin autoantibodies	These are usually present with Hashimoto thyroiditis, although less so than thyroid peroxidase autoantibodies
Magnetic resonance imaging (MRI) of the brain and pituitary gland	If central hypothyroidism is present
Thyroid ultrasonography	If a thyroid nodule is detected on examination
Fine needle aspiration	To determine whether a palpable nodule is malignant
Measurement of luteinizing hormone (LH), follicle-stimulating hormone (FSH), cortisol level, prolactin, insulin-like growth factor 1	If central hypothyroidism is diagnosed (to evaluate for hypopituitarism)
Thyrotropin-releasing hormone stimulating test	To distinguish secondary from tertiary hypothyroidism
Electrolyte measurements	Severe hypothyroidism may result in hyponatremia
Lipid panel	Hypothyroidism is associated with elevated triglycerides and elevated low-density lipoprotein cholesterol
Complete blood cell count	Hypothyroidism is associated with anemia
Fasting blood glucose measurement	To evaluate for diabetes

FURTHER READING

American Association of Clinical Endocrinologists. Medical guidelines for clinical practice for the evaluation and treatment of hyperthyroidism and hypothyroidism. Endocr Pract. 2002;8:458-469.

Devdhar M, Ousman YH, Burman KD. Hypothyroidism. Endocrinol Metab Clin North Am. 2007;36:595-615.

Hueston WJ. Treatment of hypothyroidism. Am Fam Physician. 2001;64:1717-1724.

Schmidt DN, Wallace K. How to diagnose hypopituitarism. Postgrad Med. 1998;104(7):77-78. 81-87

Wartofsky L, Van Nostrand D, Burman KD. Overt and “subclinical” hypothyroidism in women. Obstet Gynecol Surv. 2006;61:535-542.

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