The continued increase in US population, longer life expectancies, and the increase in the proportion of the population aged ≥65 years since 1950 [1], will result in Americans experiencing physician shortages of at least 91,500 in all medical specialties by 2020 [2]. An endocrinologist is a specialist in internal medicine or pediatric medicine who diagnoses and manages a wide range of diseases including diabetes, obesity, osteoporosis, and other disorders of the endocrine system. Diabetes affects 25.8 million people in the US [3] and one in three US adults are obese [4]. Clinical endocrinology is anticipated to be in greater demand by 2020 because of the larger proportion of the population with aging issues, obesity, and diabetes, and it has been suggested that this demand will exceed the capacity of the endocrinology workforce [5–8]. This prediction is supported by evidence of patients’ long wait times for the initial visit with an endocrinologist [6, 9]. The latest report from the 2012 Endocrinologist Survey showed that the average clinic waiting time was 37 days, and patients in some regions even experienced 3–6 month delays [10].

Our assessment of spatial accessibility incorporates separate notions of availability (e.g., the physical presence of endocrinologists who are available to treat patients) and accessibility (a consideration of impedance for patients to travel to the treatment locations) [11]. Accessibility, one of these dimensions for evaluating access to health care, describes geographical barriers including distance, transportation, travel time, and cost. It highlights the geographical location of services in relation to population in need [12, 13]. Geographic accessibility to endocrinologists is an important aspect of medical accessibility, which can affect the quality and timeliness of care for patients with metabolic and endocrine diseases [6]. While a study showed that the national ratio of children to pediatric endocrinologists in 2003–2004 was 290:1 for children with diabetes, and 17,741:1 for children with obesity [14], the geographic shortage in the adult endocrinologist workforce is unknown.

The main purpose of this study is to evaluate geographic coverage of, and access to, endocrinologists at different geographic levels by age group, urban/rural status, and distances for both youth and adult populations using the 2012 National Provider Identifier (NPI) Registry, which includes the service addresses of endocrinologists, and 2010 US Census block-level population data. To our knowledge, this is the first report to address geographic access to endocrinologist services at the national, state, and county levels in the United States.

Figure 2 shows two US maps of county-specific population counts for adults aged ≥18 years and for children. The locations of pediatric/adult endocrinologists are also depicted on the two age-specific maps. Endocrinologists were generally located in high-density populated areas, while urban clusters and rural areas had fewer endocrinologists. Among 6,501 adult endocrinologists, 6,201 (95.4 %) were located in urbanized areas, 176 (2.7 %) were located in urban clusters and 124 (1.9 %) were located in rural areas. Similarly among 1,203 pediatric endocrinologists, 1,165 (96.8 %) were located in urbanized areas, but only 17 (1.4 %) were located in urban clusters and 21 (1.8 %) were located in rural areas.

Within the 3,143 US counties or county-equivalents, 734 counties (whose population represented 78.7 % of the US population aged ≥ 18 years) had at least one adult endocrinologist practice and 233 (whose population represented 52.0 % of the US population aged 0–17 years) had at least one pediatric endocrinologist practice. Within 20 miles, in only 100 counties (whose population represented 17.7 % of the US population aged 0–17 years) did 100 % of the population aged 0–17 years have access to at least one pediatric endocrinologist, and in only 382 counties (whose population represented 42.4 % of the US population aged ≥18 years) did 100 % of the population aged ≥18 years have access to at least one endocrinologist. However, within a distance of 50 miles, 100 % geographic access was observed among 1,016 counties for children aged 0–17 years, 1,988 counties for adults aged 18–64 years, and 1,998 counties for adults aged ≥65 years. The number of counties without any access within 50 miles of an endocrinologist location was 1,146 (whose population represented 9.0 % of the US population aged 0–17 years) for children, 414 counties (whose population represented 1.5 % of the US population aged 18–64 years) for adults aged 18–64 years, and 419 counties (whose population represented 1.8 % of the US population aged ≥65 years) for adults aged ≥65 years.

Figure 3 shows the percentage of the age-specific populations that had access to at least one endocrinologist by urban/rural status and distance. The urbanized area adult populations (both aged 18–64 years and aged ≥65 years) had at least one endocrinologist at much shorter distances. Almost 99 % of adults living in urbanized areas had access to an adult endocrinologist within a 20-mile radius distance, while only about 53 % of the rural and 43 % of the urban cluster adults could access an adult endocrinologist within same distance. However, at a 50-mile distance, 90 % of rural adults and about 85 % of urban cluster adults had access to at least one endocrinologist. Contrasted with accessibility to adult endocrinologists, accessibility to pediatric endocrinologists was much more limited. Only about 80 % of urban, 20 % of rural, and 10 % of urban cluster children could access at least one pediatric endocrinologist within 20 miles. At a distance of 50 miles, over 94 % of urban, 66 % of rural, and 58 % of urban cluster children had access to at least one pediatric endocrinologist.

Figure 4 shows four US maps of state-specific percentages of adults aged 18–64 years and children with access to an endocrinologist within 20 miles and 50 miles. Maps for adults aged ≥65 years are not shown because they are similar to those for adults 18–64 years. Adults aged 18–64 years had a coverage rate (within 20 miles) that ranged from 37.5 % in Wyoming to 100 % in 3 of the smaller states (Connecticut, New Jersey, Rhode Island) and DC. Expanding the distance threshold to 50 miles resulted in a range of 53.4 % in Wyoming to 100 % for 12 states (Connecticut, Delaware, Indiana, Kentucky, Maryland, Massachusetts, New Jersey, Ohio, Pennsylvania, Rhode Island, South Carolina, and Virginia) and DC. In 2012, there were no pediatric endocrinologists reported by the NPI Registry data in Idaho, Montana, and Wyoming. Idaho’s pediatric endocrinologist geographic access was due to the presence of these specialists in neighboring states. However, children in Montana and Wyoming had no access within a 50-mile distance from neighboring states. The state-specific percentages of children with access to at least one pediatric endocrinologist ranged from 0 % in Montana and Wyoming to 100 % in DC within 20 miles, and from 0 % in Montana and Wyoming to 100 % in Connecticut, New Jersey, Rhode Island and DC within 50 miles.

Detailed state-specific percentages of populations with access to at least one endocrinologist, by age group and distances in the US are provided in Additional file 1.

There were 1,966 (62.6 %) of 3,143 US counties whose population had some access to an adult endocrinologist within 20 miles for adults aged 18–64 years. Among them, 382 counties had 100 % geographic access. Figure 5a shows the county-specific percentage of adults aged 18–64 years with access to at least one adult endocrinologist within 20 miles. Figure 5b shows that the ratio of adults aged 18–64 years per endocrinologist ranged greatly between those 1,966 counties from 1,571:1 to 115,685:1 and greater.

Figure 6a shows the county-specific percentages of children with access to at least one pediatric endocrinologist and Fig. 6b shows the county-specific ratios of children to pediatric endocrinologist within 20 miles.

Additional file 2 shows the county-specific percentage of adults aged ≥ 65 years with access to at least one adult endocrinologist within 20 miles.

By using the 2012 NPI Registry data and 2010 US census data linked with geographic locations, we provide a broad picture of the geographic access of the endocrinology workforce throughout the US at many different access distances. We found that 35.9 % of children, 14.6 % of adults aged 18–64 years, and 17.9 % of adults aged ≥65 years did not have access to any endocrinologist within a 20-mile radius. According to the US 2010 census, 71.2 % of the US population lives in an urbanized area, 9.5 % lives in urban clusters and 19.3 % lives in rural areas [19]. In urbanized areas, almost 99 % of US adults had access to at least one endocrinologist and 82 % of children had access to at least one pediatric endocrinologist within 20-miles of their census block centroid. However, adults and children living in rural areas and urban clusters required longer travel distances to gain access. Within 50 miles, about 90 % of adults in rural areas and 85 % in urban clusters had access to an endocrinologist but only about 65 % of children in rural areas and 60 % in urban clusters had access to a pediatric endocrinologist. These results were interesting in that populations living in urban clusters had lower spatial accessibility than populations living in rural areas. This is due to the nature of endocrinologist geographic distributions and the fact that urban clusters tend to be relatively far away from urbanized areas where most endocrinologists practice. Rural area populations that were near urbanized areas and urban clusters had spatial access to endocrinologists that practiced in them. Meanwhile, we found that urban cluster populations had less access to urbanized area endocrinologists. The population weighted share of endocrinologists increased more for rural areas than for urban clusters. These results also confirm earlier findings that there are considerable geographic disparities in the supply of pediatric endocrinologists [8, 14]. For each urban/rural area and distance, children had lower access to a pediatric endocrinologist in contrast to adult access to an adult endocrinologist. Because children are less likely to have endocrine diseases, the shortage may not be severe. Accessibility for both adults and children varied by state and county. Our findings on geographic access to endocrinologists may provide valuable information for medical education and health resources allocation.

The coverage approach using defined buffer zones in this study avoids the problem that arises when aggregating data according to existing census geographic units (e.g., states, counties), which vary greatly in size and do not necessarily contain homogeneous populations [26]. Our method also accounts for potential cross-boundary (county and state) health care seeking behaviors in the US. Traditional approaches for estimating geographic access of physicians generally are limited to those physician practices in a specific area, such as a county. That simple and intuitive traditional approach inappropriately assumes no cross-border health-providing or health-seeking behaviors outside of that geographic area. However, people very often seek health service by crossing political boundaries such as state or county lines especially when residing in smaller geographic areas, isolated rural areas, or in residences near state or county borders.

Our general approach is similar to the two-step floating catchment area (2SFCA) method of Luo and Wang [20]. In the 2SFCA method, most commonly a single driving time distance between physician practice locations and populations is used to define the catchment area (buffer zone). We explored the multiple buffer distance zones to explore the sensitivity of spatial access coverage to multiple distance search boundaries. Also, we used Euclidian distance to create a buffer zone (catchment area) because of the significant computation time for a national level study, and because for a non-emergency service, both straight-line distance and drive time/distance provide similar precision [27]. By using our coverage approach, we virtually divided an endocrinologist by the population within that zone as a share (i.e., a physician-to-population ratio for the physician) and distributed a share to each person living in that circular buffer zone. The total accessibility for an individual to any physicians is the total shares connected to that individual. In this way, the accessibility of a geographic area is estimated by the total population in that area divided by the sum of the total shares of all individuals in the interested area. As with other FCA methods, our approach allows flexibility in computing estimates for different geographic levels and for calculating large nation-wide estimates. We did not overcome the subjectivity of a predefined distance search boundary like all the family of 2SFCA methods. The distance decay function is applied to some modified 2SFCA methods to account for the unequal accessibility to physicians within the single predefined distance searching boundary or travelable distance limit. Like classic 2SFCA, we did not consider the unequal probability of access to endocrinologists within a predefined distance buffer zone, because to our knowledge, for non-emergency care access to physicians, distance decay was limited within a travelable distance or predefined distance buffer zone in this study. Since we do not have an informed knowledge of travelable distances that would work for our entire study area, we chose to implement the conceptually simpler approach of using multiple distance buffers as a sensitivity analysis.

Endocrinologists are a crucial part of the health care network required to meet the growing need for the diagnosis and management of complex cases of obesity-related diseases such as diabetes and metabolic syndrome. Unfortunately, the prevalence of obesity in the US has increased three-fold among children and nearly as greatly among adults during the past three decades to become a critical public health problem [4, 28]. As of 2011–2012, the prevalence of obesity remained high among children aged 2–5 years (8.4 %), aged 6–11 years (17.7 %), and aged 12–19 years (20.5 %) [29]. Likewise, more than one-third (35.4 %) of US adults aged ≥20 years were obese during the same time period [4]. Concurrent with the rise in obesity, the prevalence of diagnosed diabetes among adults has also increased from 3.5 % in 1980 to 9.0 % in 2011 [30]. Among children, a 2009 study estimated that there were 187,000 children with diabetes in the US who were potentially in need of consultation with a pediatric endocrinologist [31]. Future studies would benefit in estimating geographic access of obese and diabetic adult populations to endocrinologists and in updating the seminal work by Lee et al., which used data from the 2003–2004 National Survey of Children’s Health to estimate the ratios of diabetic children and obese children to the 2004 supply of pediatric endocrinologists in the US [14].

There are some limitations in this study. First, the NPI Registry data from the Centers for Medicare and Medicaid Services may include more endocrinology practices than other physician workforce datasets because of the HIPAA requirement that all covered health care providers must obtain an NPI [15]. Other reports on issues related to the endocrinologist workforce have used the American Medical Association Masterfile [5, 9, 10], American Board of Pediatrics records [14], American Board of Internal Medicine records [6, 7], and membership records of The Endocrine Society [32]. Therefore, our estimates may overestimate actual numbers of endocrinologists in practice. Second, we have not accounted for board certification status, excluded retired or inactive professionals, or identified the major professional activity (i.e., proportion of time spent in office-based practice, hospital staff, research, teaching, or administrative duties, multiple practice) as other reports have done [5, 14]. Vigersky and colleagues [10] estimated that there were 5,496 board certified adult and 1,016 pediatric endocrinologists in 2011. Furthermore, because NPI only included one practice location for physicians, we were unable to include endocrinologists’ multiple practice locations in our analysis. Therefore, we may have either under- or over-estimated the geographic accessibility to endocrinologists in this study. Third, since we used Euclidean distance as a proxy for travel distance, our metric may overestimate the geographic accessibility to an endocrinologist, especially in rural or mountain areas where road networks are more limited than in urban areas. Actual travel distances or travel times using network analysis may be able to improve precision. However, for a national study, it would be computationally prohibitive at present to analyze coverage using detailed street network data. In addition, a recent analysis showed that for nonemergency travel to hospitals, the added precision of using street networks is inconsequential [27].

A further limitation is that it is possible that there are important variations in practice patterns of endocrinologists and primary care physicians who treat patients with diabetes and obesity. Such practice variations could mitigate lack of access to endocrinologists, particularly in areas or cases where less severe presentations are treated by primary care physicians and/or other health specialists, such as dietitians. Information on practice variations could be useful in further research on the association between geographic variations in access to care and health outcomes. In addition, like classic 2SFCA, we did not consider the unequal probability access to endocrinologists within a predefined distance buffer zone. We assumed that for the non-emergent care access, distance decay was limited within a predefined buffer zone.

A clear picture of the current geographic access to endocrinologists in the US is important for resource planning and allocation and intervention strategy development. Residents of urban clusters and rural areas are likely to continue to travel greater distances to reach an endocrinologist. According to Vigersky [10] 85 % of diabetes care was performed by health care providers other than endocrinologists (e.g., primary care physicians, physician assistants, nurse practitioners). One solution that has been recommended to fill the gaps in geographic access, particularly for children with diabetes, is to promote the development and/or improvement of telemedicine opportunities so that primary care providers can manage the patient but consult with specialists [9].

There were substantial nationwide variations in geographic accessibility to pediatric and adult endocrinologists at county and state levels, as well as disparities in access to an endocrinologist within a reasonable driving distance, by urban/rural status and by age. Our findings show an unequal geographic distribution, including shortages, of the endocrine workforce in the US, and highlight areas with limited endocrinologist resources. Given the backdrop of increases in obesity and diabetes, understanding that geographic access to one of the types of health care providers (endocrinologists) who treat these conditions can inform future research by suggesting inclusion of geographic access to care as a potential explanatory variable for variations in obesity and diabetes. Our approach can be used to further assess the impact of geographic variations in access to endocrinologists on disease detection, prevention, and control. These methods also can be applied to other health care workforce spatial accessibility and coverage analyses.