Arthur M. Michalek, PhD

Thyroid disease and diabetes mellitus account for considerable morbidity in the United States. The annual incidence of hypothyroidism in adults is 0.08% to 0.2%. Hypothyroidism is more common among women than men, and incidence increases with age. An estimated 6% of the US population has diabetes, and more than 600,000 new cases are diagnosed each year. Both diseases are more prevalent in women. Thyroid disease has been reported to occur with such frequency within populations with diabetes that some authorities recommend routine screening for those patients.^1-3 Hypothyroidism has been observed to be quite prevalent in patients with diabetes.^2-6 Although American Indians are known to have high prevalence rates of diabetes, no published reports have examined the occurrence of hypothyroidism among American Indians with diabetes.

Methods

A medical record review was undertaken for all patients attending the health clinic of a northeastern American Indian tribe. This tribal community is located in a rural area that is approximately 60 miles from major medical services. The Tribal Health Center is operated as a local comprehensive health care delivery system under contract with the Indian Health Service (IHS). The IHS, an agency of the Department of Health and Human Services, provides funding and oversight to federally recognized tribes for a broad-spectrum program of preventive, curative, rehabilitative, and environmental services.⁷ This system integrates health services provided directly through IHS facilities, purchased by IHS through contractual arrangements with providers in the private sector, and delivered through programs operated by tribes. The main objective of the health center is to provide the highest level of health care possible for eligible recipients in the service delivery area. The center is staffed by full-time physicians, physician assistants, nurse practitioners, pharmacists, counselors, aides, and clerical and administrative personnel.

Although patient medical records are maintained in paper form, key elements of the patient visit, including diagnosis, are entered into a central IHS patient care database. We used this clinic database to identify diagnostic groups for our study. A medical record review was undertaken for the 892 individuals (415 men, 477 women) eligible for clinic services during calendar year 1998. These individuals represent the denominator and are the number of eligible American Indians living within the geographic service area of the clinic. Of the 477 women, 431 were aged younger than 60 years. We directed our attention to patient records listed with a diagnosis of either diabetes mellitus or hypothyroidism. Odds ratios (ORs) were computed, and 95% confidence intervals (CIs) were calculated as described by Rothman.⁸

Results

An electronic search of clinic records for 1998 identified a total of 156 cases of diabetes (17.5% prevalence) and 25 cases of clinical hypothyroidism (2.8%) occurring among the 892 eligible patients ([Table 1]). Among men (n=415) there were 54 cases of diabetes (13%) and 1 case of hypothyroidism (0.2%). The 1 case of hypothyroidism occurred in a man with diabetes. For women (n=477) prevalence rates of diabetes and hypothyroidism were higher, as expected, with a total of 102 cases of diabetes (21.4%) and 24 cases of hypothyroidism (5%).

Prevalence rates for hypothyroidism differed markedly by age and diabetes status. The prevalence of hypothyroidism among women with diabetes was 8.8% overall, ranging from 5% among women younger than 60 years to 21% among women 60 years and older ([Table 2]). The OR for hypothyroidism occurring among women with diabetes relative to those who did not have diabetes was 2.32 (95% CI, 0.9-5.8). Also, the ORs of these 2 conditions among women appeared to be modified by age, with the OR being 1.4 (95% CI, 0.4-4.8) among women younger than 60 years and 2.63 (95% CI, 0.4-22.6) among those aged 60 years and older. However, these data should be viewed cautiously, because the CIs include 1.0, and because of the small number of women aged 60 years or older.

Discussion

Although our paper is unique because we document increased prevalence of hypothyroidism specifically in an American Indian population with diabetes, it is consistent with the general literature. The prevalence of hypothyroidism among people with diabetes ranges from 0.2% to 6% depending on age and sex.¹ Ganz and Kozak² reviewed the records of 60,703 patients with diabetes from 1957 to 1972 and reported finding 114 (0.19%) cases of hypothyroidism. Hecht and Gershberg³ reported on 9 (1.7%) hypothyroid patients out of 530 patients with diabetes who attended their metabolism clinic. Perros⁴ reported a prevalence rate of 13.4% for all types of thyroid disease among a population with diabetes receiving annual thyroid screening.⁴ Smithson,⁵ reporting on thyroid screening of 197 patients with diabetes in a general practice setting, noted that 6% of the women with diabetes also had hypothyroidism. Feely and Isles⁶ reported a 4% prevalence of clinical hypothyroidism in a screened population with diabetes. They also noted an increased prevalence (6%) in women with diabetes who were older than 60 years.

American Indian populations are known to have the highest rates of diabetes mellitus in the world. IHS data demonstrate a 3-fold increased mortality from diabetes among American Indians compared with the general US white population.⁷ The prevalence of diabetes in American Indians is 3 times the prevalence among non-Hispanic whites.⁹ The highest reported rates have been observed among the Pima Indians of Arizona: Approximately 70% of those aged 55 to 64 years have diabetes.¹⁰

Limitations

Although our finding of a heightened prevalence of hypothyroidism and diabetes among an American Indian population was not statistically significant, its magnitude was striking. Although our study benefited from a reliable record system and a well-defined population, we lacked a large enough population to detect statistical significance. Our results tend toward the high end of those reported in the medical literature and are most likely an underestimate of true risk. Cases of hypothyroidism and diabetes in our report represent those that were clinically apparent and discovered through a retrospective record review. They are not the result of active screening, such as that employed in some studies.^5,6 Also, we have reason to suspect that our denominator data may represent an overestimate. Denominator data are based on the number of eligible American Indians (n=892) living within the geographic service area of the clinic. Not all of these individuals rely on the clinic for health care, so they contribute to the denominator but may not add to the numerator. Another limitation of a descriptive study such as this is that we only have information on identified cases of hypothyroidism and diabetes; we have no data on confounding factors, such as obesity, family history, and so forth.

The reasons for a high concordance between diabetes and hypothyroidism are unclear. Smithson⁵ supports the theory that the high prevalence of abnormal thyroid function tests might result from the prevalence of thyroid antibodies in patients with diabetes and the influence of poorly controlled diabetes on thyroid hormone concentrations. Others support the idea that hypothyroidism and diabetes mellitus have autoimmune features.^2,3 The greater prevalence of both diseases among women is also puzzling. Both diseases may involve sex-related susceptibility genes that reside in close proximity. Perhaps both diseases are expressed with greater frequency after the altered immune states of pregnancy. These are among many reasons that may offer partial explanations for the observed relationship. Future advances in molecular biologic techniques, immunology, and human genetics combined with rigorous epidemiological assessments should clarify our observations.

Conclusions

Our findings support the need for further investigation of the association between diabetes and hypothyroidism in American Indian populations, particularly among those with known high prevalence rates of diabetes. Our observation of the association, the first reported in an American Indian population, may be of particular interest to family physicians and other clinicians providing care to these populations. However, it should be noted that our findings, although highly suggestive, were not statistically significant. Further investigations in a larger population are warranted. Also, although American Indian populations in general are perceived to have high rates of diabetes, there is extreme heterogeneity across tribal groups with respect to health conditions, genetic influences, and environmental exposure. Hypothyroidism and diabetes share clinical signs and symptoms, such as fatigue, lethargy and weight gain. Populations with diabetes experience very high rates of morbidity and mortality from a variety of disease conditions. The ability to diagnose and treat unsuspected hypothyroidism in these populations may greatly enhance quality of life.

Thyroid disease and diabetes mellitus account for considerable morbidity in the United States. The annual incidence of hypothyroidism in adults is 0.08% to 0.2%. Hypothyroidism is more common among women than men, and incidence increases with age. An estimated 6% of the US population has diabetes, and more than 600,000 new cases are diagnosed each year. Both diseases are more prevalent in women. Thyroid disease has been reported to occur with such frequency within populations with diabetes that some authorities recommend routine screening for those patients.^1-3 Hypothyroidism has been observed to be quite prevalent in patients with diabetes.^2-6 Although American Indians are known to have high prevalence rates of diabetes, no published reports have examined the occurrence of hypothyroidism among American Indians with diabetes.

Methods

A medical record review was undertaken for all patients attending the health clinic of a northeastern American Indian tribe. This tribal community is located in a rural area that is approximately 60 miles from major medical services. The Tribal Health Center is operated as a local comprehensive health care delivery system under contract with the Indian Health Service (IHS). The IHS, an agency of the Department of Health and Human Services, provides funding and oversight to federally recognized tribes for a broad-spectrum program of preventive, curative, rehabilitative, and environmental services.⁷ This system integrates health services provided directly through IHS facilities, purchased by IHS through contractual arrangements with providers in the private sector, and delivered through programs operated by tribes. The main objective of the health center is to provide the highest level of health care possible for eligible recipients in the service delivery area. The center is staffed by full-time physicians, physician assistants, nurse practitioners, pharmacists, counselors, aides, and clerical and administrative personnel.

Although patient medical records are maintained in paper form, key elements of the patient visit, including diagnosis, are entered into a central IHS patient care database. We used this clinic database to identify diagnostic groups for our study. A medical record review was undertaken for the 892 individuals (415 men, 477 women) eligible for clinic services during calendar year 1998. These individuals represent the denominator and are the number of eligible American Indians living within the geographic service area of the clinic. Of the 477 women, 431 were aged younger than 60 years. We directed our attention to patient records listed with a diagnosis of either diabetes mellitus or hypothyroidism. Odds ratios (ORs) were computed, and 95% confidence intervals (CIs) were calculated as described by Rothman.⁸

Results

An electronic search of clinic records for 1998 identified a total of 156 cases of diabetes (17.5% prevalence) and 25 cases of clinical hypothyroidism (2.8%) occurring among the 892 eligible patients ([Table 1]). Among men (n=415) there were 54 cases of diabetes (13%) and 1 case of hypothyroidism (0.2%). The 1 case of hypothyroidism occurred in a man with diabetes. For women (n=477) prevalence rates of diabetes and hypothyroidism were higher, as expected, with a total of 102 cases of diabetes (21.4%) and 24 cases of hypothyroidism (5%).

Prevalence rates for hypothyroidism differed markedly by age and diabetes status. The prevalence of hypothyroidism among women with diabetes was 8.8% overall, ranging from 5% among women younger than 60 years to 21% among women 60 years and older ([Table 2]). The OR for hypothyroidism occurring among women with diabetes relative to those who did not have diabetes was 2.32 (95% CI, 0.9-5.8). Also, the ORs of these 2 conditions among women appeared to be modified by age, with the OR being 1.4 (95% CI, 0.4-4.8) among women younger than 60 years and 2.63 (95% CI, 0.4-22.6) among those aged 60 years and older. However, these data should be viewed cautiously, because the CIs include 1.0, and because of the small number of women aged 60 years or older.

Discussion

Although our paper is unique because we document increased prevalence of hypothyroidism specifically in an American Indian population with diabetes, it is consistent with the general literature. The prevalence of hypothyroidism among people with diabetes ranges from 0.2% to 6% depending on age and sex.¹ Ganz and Kozak² reviewed the records of 60,703 patients with diabetes from 1957 to 1972 and reported finding 114 (0.19%) cases of hypothyroidism. Hecht and Gershberg³ reported on 9 (1.7%) hypothyroid patients out of 530 patients with diabetes who attended their metabolism clinic. Perros⁴ reported a prevalence rate of 13.4% for all types of thyroid disease among a population with diabetes receiving annual thyroid screening.⁴ Smithson,⁵ reporting on thyroid screening of 197 patients with diabetes in a general practice setting, noted that 6% of the women with diabetes also had hypothyroidism. Feely and Isles⁶ reported a 4% prevalence of clinical hypothyroidism in a screened population with diabetes. They also noted an increased prevalence (6%) in women with diabetes who were older than 60 years.

American Indian populations are known to have the highest rates of diabetes mellitus in the world. IHS data demonstrate a 3-fold increased mortality from diabetes among American Indians compared with the general US white population.⁷ The prevalence of diabetes in American Indians is 3 times the prevalence among non-Hispanic whites.⁹ The highest reported rates have been observed among the Pima Indians of Arizona: Approximately 70% of those aged 55 to 64 years have diabetes.¹⁰

Limitations

Although our finding of a heightened prevalence of hypothyroidism and diabetes among an American Indian population was not statistically significant, its magnitude was striking. Although our study benefited from a reliable record system and a well-defined population, we lacked a large enough population to detect statistical significance. Our results tend toward the high end of those reported in the medical literature and are most likely an underestimate of true risk. Cases of hypothyroidism and diabetes in our report represent those that were clinically apparent and discovered through a retrospective record review. They are not the result of active screening, such as that employed in some studies.^5,6 Also, we have reason to suspect that our denominator data may represent an overestimate. Denominator data are based on the number of eligible American Indians (n=892) living within the geographic service area of the clinic. Not all of these individuals rely on the clinic for health care, so they contribute to the denominator but may not add to the numerator. Another limitation of a descriptive study such as this is that we only have information on identified cases of hypothyroidism and diabetes; we have no data on confounding factors, such as obesity, family history, and so forth.

The reasons for a high concordance between diabetes and hypothyroidism are unclear. Smithson⁵ supports the theory that the high prevalence of abnormal thyroid function tests might result from the prevalence of thyroid antibodies in patients with diabetes and the influence of poorly controlled diabetes on thyroid hormone concentrations. Others support the idea that hypothyroidism and diabetes mellitus have autoimmune features.^2,3 The greater prevalence of both diseases among women is also puzzling. Both diseases may involve sex-related susceptibility genes that reside in close proximity. Perhaps both diseases are expressed with greater frequency after the altered immune states of pregnancy. These are among many reasons that may offer partial explanations for the observed relationship. Future advances in molecular biologic techniques, immunology, and human genetics combined with rigorous epidemiological assessments should clarify our observations.

Conclusions

Our findings support the need for further investigation of the association between diabetes and hypothyroidism in American Indian populations, particularly among those with known high prevalence rates of diabetes. Our observation of the association, the first reported in an American Indian population, may be of particular interest to family physicians and other clinicians providing care to these populations. However, it should be noted that our findings, although highly suggestive, were not statistically significant. Further investigations in a larger population are warranted. Also, although American Indian populations in general are perceived to have high rates of diabetes, there is extreme heterogeneity across tribal groups with respect to health conditions, genetic influences, and environmental exposure. Hypothyroidism and diabetes share clinical signs and symptoms, such as fatigue, lethargy and weight gain. Populations with diabetes experience very high rates of morbidity and mortality from a variety of disease conditions. The ability to diagnose and treat unsuspected hypothyroidism in these populations may greatly enhance quality of life.

Thyroid disease and diabetes mellitus account for considerable morbidity in the United States. The annual incidence of hypothyroidism in adults is 0.08% to 0.2%. Hypothyroidism is more common among women than men, and incidence increases with age. An estimated 6% of the US population has diabetes, and more than 600,000 new cases are diagnosed each year. Both diseases are more prevalent in women. Thyroid disease has been reported to occur with such frequency within populations with diabetes that some authorities recommend routine screening for those patients.^1-3 Hypothyroidism has been observed to be quite prevalent in patients with diabetes.^2-6 Although American Indians are known to have high prevalence rates of diabetes, no published reports have examined the occurrence of hypothyroidism among American Indians with diabetes.

Methods

A medical record review was undertaken for all patients attending the health clinic of a northeastern American Indian tribe. This tribal community is located in a rural area that is approximately 60 miles from major medical services. The Tribal Health Center is operated as a local comprehensive health care delivery system under contract with the Indian Health Service (IHS). The IHS, an agency of the Department of Health and Human Services, provides funding and oversight to federally recognized tribes for a broad-spectrum program of preventive, curative, rehabilitative, and environmental services.⁷ This system integrates health services provided directly through IHS facilities, purchased by IHS through contractual arrangements with providers in the private sector, and delivered through programs operated by tribes. The main objective of the health center is to provide the highest level of health care possible for eligible recipients in the service delivery area. The center is staffed by full-time physicians, physician assistants, nurse practitioners, pharmacists, counselors, aides, and clerical and administrative personnel.

Although patient medical records are maintained in paper form, key elements of the patient visit, including diagnosis, are entered into a central IHS patient care database. We used this clinic database to identify diagnostic groups for our study. A medical record review was undertaken for the 892 individuals (415 men, 477 women) eligible for clinic services during calendar year 1998. These individuals represent the denominator and are the number of eligible American Indians living within the geographic service area of the clinic. Of the 477 women, 431 were aged younger than 60 years. We directed our attention to patient records listed with a diagnosis of either diabetes mellitus or hypothyroidism. Odds ratios (ORs) were computed, and 95% confidence intervals (CIs) were calculated as described by Rothman.⁸

Results

An electronic search of clinic records for 1998 identified a total of 156 cases of diabetes (17.5% prevalence) and 25 cases of clinical hypothyroidism (2.8%) occurring among the 892 eligible patients ([Table 1]). Among men (n=415) there were 54 cases of diabetes (13%) and 1 case of hypothyroidism (0.2%). The 1 case of hypothyroidism occurred in a man with diabetes. For women (n=477) prevalence rates of diabetes and hypothyroidism were higher, as expected, with a total of 102 cases of diabetes (21.4%) and 24 cases of hypothyroidism (5%).

Prevalence rates for hypothyroidism differed markedly by age and diabetes status. The prevalence of hypothyroidism among women with diabetes was 8.8% overall, ranging from 5% among women younger than 60 years to 21% among women 60 years and older ([Table 2]). The OR for hypothyroidism occurring among women with diabetes relative to those who did not have diabetes was 2.32 (95% CI, 0.9-5.8). Also, the ORs of these 2 conditions among women appeared to be modified by age, with the OR being 1.4 (95% CI, 0.4-4.8) among women younger than 60 years and 2.63 (95% CI, 0.4-22.6) among those aged 60 years and older. However, these data should be viewed cautiously, because the CIs include 1.0, and because of the small number of women aged 60 years or older.

Discussion

Although our paper is unique because we document increased prevalence of hypothyroidism specifically in an American Indian population with diabetes, it is consistent with the general literature. The prevalence of hypothyroidism among people with diabetes ranges from 0.2% to 6% depending on age and sex.¹ Ganz and Kozak² reviewed the records of 60,703 patients with diabetes from 1957 to 1972 and reported finding 114 (0.19%) cases of hypothyroidism. Hecht and Gershberg³ reported on 9 (1.7%) hypothyroid patients out of 530 patients with diabetes who attended their metabolism clinic. Perros⁴ reported a prevalence rate of 13.4% for all types of thyroid disease among a population with diabetes receiving annual thyroid screening.⁴ Smithson,⁵ reporting on thyroid screening of 197 patients with diabetes in a general practice setting, noted that 6% of the women with diabetes also had hypothyroidism. Feely and Isles⁶ reported a 4% prevalence of clinical hypothyroidism in a screened population with diabetes. They also noted an increased prevalence (6%) in women with diabetes who were older than 60 years.

American Indian populations are known to have the highest rates of diabetes mellitus in the world. IHS data demonstrate a 3-fold increased mortality from diabetes among American Indians compared with the general US white population.⁷ The prevalence of diabetes in American Indians is 3 times the prevalence among non-Hispanic whites.⁹ The highest reported rates have been observed among the Pima Indians of Arizona: Approximately 70% of those aged 55 to 64 years have diabetes.¹⁰

Limitations

Although our finding of a heightened prevalence of hypothyroidism and diabetes among an American Indian population was not statistically significant, its magnitude was striking. Although our study benefited from a reliable record system and a well-defined population, we lacked a large enough population to detect statistical significance. Our results tend toward the high end of those reported in the medical literature and are most likely an underestimate of true risk. Cases of hypothyroidism and diabetes in our report represent those that were clinically apparent and discovered through a retrospective record review. They are not the result of active screening, such as that employed in some studies.^5,6 Also, we have reason to suspect that our denominator data may represent an overestimate. Denominator data are based on the number of eligible American Indians (n=892) living within the geographic service area of the clinic. Not all of these individuals rely on the clinic for health care, so they contribute to the denominator but may not add to the numerator. Another limitation of a descriptive study such as this is that we only have information on identified cases of hypothyroidism and diabetes; we have no data on confounding factors, such as obesity, family history, and so forth.

The reasons for a high concordance between diabetes and hypothyroidism are unclear. Smithson⁵ supports the theory that the high prevalence of abnormal thyroid function tests might result from the prevalence of thyroid antibodies in patients with diabetes and the influence of poorly controlled diabetes on thyroid hormone concentrations. Others support the idea that hypothyroidism and diabetes mellitus have autoimmune features.^2,3 The greater prevalence of both diseases among women is also puzzling. Both diseases may involve sex-related susceptibility genes that reside in close proximity. Perhaps both diseases are expressed with greater frequency after the altered immune states of pregnancy. These are among many reasons that may offer partial explanations for the observed relationship. Future advances in molecular biologic techniques, immunology, and human genetics combined with rigorous epidemiological assessments should clarify our observations.

Conclusions

Our findings support the need for further investigation of the association between diabetes and hypothyroidism in American Indian populations, particularly among those with known high prevalence rates of diabetes. Our observation of the association, the first reported in an American Indian population, may be of particular interest to family physicians and other clinicians providing care to these populations. However, it should be noted that our findings, although highly suggestive, were not statistically significant. Further investigations in a larger population are warranted. Also, although American Indian populations in general are perceived to have high rates of diabetes, there is extreme heterogeneity across tribal groups with respect to health conditions, genetic influences, and environmental exposure. Hypothyroidism and diabetes share clinical signs and symptoms, such as fatigue, lethargy and weight gain. Populations with diabetes experience very high rates of morbidity and mortality from a variety of disease conditions. The ability to diagnose and treat unsuspected hypothyroidism in these populations may greatly enhance quality of life.