Hypothyroidism 

  • Author: Shikha Bharaktiya, MD; Chief Editor: George T Griffing, MD   more...
 
Updated: Jul 22, 2011
 

Background

Hypothyroidism is a common endocrine disorder resulting from deficiency of thyroid hormone. It usually is a primary process in which the thyroid gland produces insufficient amounts of thyroid hormone. It can also be secondary—that is, lack of thyroid hormone secretion due to inadequate secretion of either thyrotropin (ie, thyroid-stimulating hormone [TSH]) from the pituitary gland or thyrotropin-releasing hormone (TRH) from the hypothalamus (secondary or tertiary hypothyroidism). The patient's presentation may vary from asymptomatic to, rarely, coma with multisystem organ failure (myxedema coma). The most common cause in the Unites States is autoimmune thyroid disease (Hashimoto thyroiditis).

Cretinism refers to congenital hypothyroidism, which affects 1 per 4000 newborns.

Subclinical hypothyroidism, also referred to as mild hypothyroidism, is defined as normal serum free T4 levels with slightly high serum TSH concentration.

Recent studies

In a 12-year longitudinal study, Stuckey et al investigated the long-term risk of hypothyroidism in women who previously had had postpartum thyroid dysfunction (PPTD). The study involved 409 women, 71 of whom had previously been diagnosed with PPTD. At 12-year follow-up, 27 women in the PPTD group and 14 women in the non-PPTD group (38% and 4%, respectively) were found to have hypothyroidism. Based on an analysis of odds ratios (ORs), the authors concluded that within the PPTD group, women who had been diagnosed with postpartum hypothyroidism were among those at particularly high long-term risk for hypothyroidism (OR = 9.7).[1]

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Pathophysiology

Localized disease of the thyroid gland that results in decreased thyroid hormone production is the most common cause of hypothyroidism. Under normal circumstances, the thyroid releases 100-125 nmol of thyroxine (T4) daily and only small amounts of triiodothyronine (T3). The half-life of T4 is approximately 7-10 days. T4, a prohormone, is converted to T3, the active form of thyroid hormone, in the peripheral tissues by 5’-deiodination. Early in the disease process, compensatory mechanisms maintain T3 levels. Decreased production of T4 causes an increase in the secretion of TSH by the pituitary gland. TSH stimulates hypertrophy and hyperplasia of the thyroid gland and thyroid T4-5'-deiodinase activity. This, in turn, causes the thyroid to release more T3.

Because all metabolically active cells require thyroid hormone, deficiency of the hormone has a wide range of effects. Systemic effects are due to either derangements in metabolic processes or direct effects by myxedematous infiltration (ie, accumulation of glucosaminoglycans in the tissues).

The myxedematous changes in the heart result in decreased contractility, cardiac enlargement, pericardial effusion, decreased pulse, and decreased cardiac output. In the GI tract, achlorhydria and decreased intestinal transit with gastric stasis can occur. Delayed puberty, anovulation, menstrual irregularities, and infertility are common. Decreased thyroid hormone effect can cause increased levels of total cholesterol and low-density lipoprotein (LDL) cholesterol and a possible change in high-density lipoprotein (HDL) cholesterol due to a change in metabolic clearance. In addition, hypothyroidism may result in an increase in insulin resistance.

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Epidemiology

Frequency

United States

The National Health and Nutrition Examination Survey (NHANES 1999-2002) of 4,392 individuals reflecting the US population reported hypothyroidism (defined as TSH levels >4.5 mIU/L) in 3.7% of the population.[2] Hypothyroidism is more common in women with small body size at birth and low body mass index during childhood.[3]

International

Iodine deficiency as a cause of hypothyroidism is more common internationally. The prevalence is reported as 2-5% depending on the study, increasing to 15% by age 75 years.

Mortality/Morbidity

In developed countries, death caused by hypothyroidism is uncommon.

Race

NHANES 1999-2002 reported that the prevalence of hypothyroidism (including subclinical) was higher in whites (5.1%) and Mexican Americans than in African Americans (1.7%). African Americans tend to have lower TSH values.[2]

Sex

Community studies use slightly different criteria for determining hypothyroidism; therefore, female-to-male ratios vary. Generally, thyroid disease is much more common in females than in males, with reports of prevalence 2-8 times higher in females.

Age

The frequency of hypothyroidism, goiters, and thyroid nodules increases with age. Hypothyroidism is most prevalent in elderly populations, with 2% to as much as 20% of older age groups having some form of hypothyroidism. The Framingham study found hypothyroidism (TSH >10 mIU/L) in 5.9% of women and 2.4% of men older than 60 years.[4] In NHANES 1999-2002, the odds of having hypothyroidism were 5 times greater in persons aged 80 years and older than in individuals aged 12-49 years.[2]

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Contributor Information and Disclosures
Author

Shikha Bharaktiya, MD  Clinical Fellow, Department of Internal Medicine, Division of Endocrinology and Metabolism, University of Texas Medical School at Houston

Disclosure: Nothing to disclose.

Coauthor(s)

Philip R Orlander, MD  Assistant Dean for Educational Affairs, Vice-Chair of Medicine for Education, Director and Professor, Division of Endocrinology, University of Texas Health Science Center at Houston

Philip R Orlander, MD is a member of the following medical societies: American Association of Clinical Endocrinologists, American Diabetes Association, Endocrine Society, and Texas Medical Association

Disclosure: Nothing to disclose.

Walter R Woodhouse, MD, MSA  Associate Clinical Professor, Department of Family Practice, Medical College of Ohio

Walter R Woodhouse, MD, MSA is a member of the following medical societies: American Academy of Family Physicians, American Academy of Pain Medicine, and Society of Teachers of Family Medicine

Disclosure: Nothing to disclose.

Anu Bhalla Davis, MD  Assistant Professor, Department of Internal Medicine, Division of Diabetes, Endocrinology, and Metabolism, University of Texas Medical School at Houston

Disclosure: Nothing to disclose.

Specialty Editor Board

Frederick H Ziel, MD  Associate Professor of Medicine, University of California, Los Angeles, David Geffen School of Medicine; Physician-In-Charge, Endocrinology/Diabetes Center, Director of Medical Education, Kaiser Permanente Woodland Hills; Chair of Endocrinology, Co-Chair of Diabetes Complete Care Program, Southern California Permanente Medical Group

Frederick H Ziel, MD is a member of the following medical societies: American Association of Clinical Endocrinologists, American College of Endocrinology, American College of Physicians, American College of Physicians-American Society of Internal Medicine, American Diabetes Association, American Federation for Medical Research, American Medical Association, American Society for Bone and Mineral Research, California Medical Association, Endocrine Society, and International Society for Clinical Densitometry

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD  Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Arthur B Chausmer, MD, PhD, FACP, FACE, FACN, CNS  Professor of Medicine (Endocrinology, Adj), Johns Hopkins School of Medicine; Affiliate Research Professor, Bioinformatics and Computational Biology Program, School of Computational Sciences, George Mason University; Principal, C/A Informatics, LLC

Arthur B Chausmer, MD, PhD, FACP, FACE, FACN, CNS is a member of the following medical societies: American Association of Clinical Endocrinologists, American College of Endocrinology, American College of Nutrition, American College of Physicians, American College of Physicians-American Society of Internal Medicine, American Medical Informatics Association, American Society for Bone and Mineral Research, Endocrine Society, and International Society for Clinical Densitometry

Disclosure: Nothing to disclose.

Mark Cooper, MBBS, PhD, FRACP  Head, Diabetes & Metabolism Division, Baker Heart Research Institute, Professor of Medicine, Monash University

Disclosure: Nothing to disclose.

Chief Editor

George T Griffing, MD  Professor of Medicine, St Louis University School of Medicine

George T Griffing, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Medical Practice Executives, American College of Physician Executives, American College of Physicians, American Diabetes Association, American Federation for Medical Research, American Heart Association, Central Society for Clinical Research, Endocrine Society, International Society for Clinical Densitometry, and Southern Society for Clinical Investigation

Disclosure: Nothing to disclose.

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