Hypothyroidism is a common condition, with a prevalence of overt and subclinical disease of 0·2 and 5·3%, respectively, and its prevalence is rising (1). Unfortunately, in real life, almost half of the patients do not achieve therapeutic targets (2). A serum TSH outside the normal range while on thyroxine (L-T4) replacement is associated with increased morbidity and mortality (3). Therefore, a significant burden of disease is associated with hypothyroidism, which can be addressed with very cheap interventions (adjustment of L-T4 dose and additional monitoring). However, implementation is vitally dependent on appropriate medical supervision. Failure to achieve a normal serum TSH is a common reason for hypothyroid patients on L-T4 being referred to thyroid specialists (4, 5).

The available information on the characteristics of specialists who treat patients with hypothyroidism in Europe is scanty. A survey of members of the European Society of Endocrinology (ESE) revealed that thyroid disorders comprised 28% of the endocrinologist workload (6). Details regarding the characteristics of specialists treating thyroid diseases were unavailable, and have not emerged since. Furthermore, it is notable that in addition to endocrinologists, clinicians from several other specialities not represented in the ESE survey treat patients with hypothyroidism in Europe. Regional differences in managing some thyroid conditions have been reported (7). However, hypothyroidism has not been studied in this context except for one previous survey, which found that American physicians were more likely to prescribe L-T3 and desiccated thyroid extract (DTE) than their European counterparts (8). The role of thyroid specialists in managing this wave of demands on healthcare systems is likely to be important. Also, primary care physicians tend to follow the lead of specialists (9). Europe comprises countries with differences in healthcare provision, patient demographics, epidemiology of thyroid disease, and gross national income per capita (GNI per capita); therefore, further exploration of these variables can help understand, plan, and improve healthcare delivery (10).

THESIS (“Treatment of Hypothyroidism in Europe by Specialists: An International Survey”) was a survey of European specialists treating patients with hypothyroidism. One of the study aims was to document the demographic and work-related characteristics of specialists who treat hypothyroid patients in European countries. Twenty out of twenty eight countries surveyed have already reported their national data (11–30). Differences in healthcare delivery may impact the patient experience, and therefore comparisons between regions and countries may provide valuable insights. Here, we present the aggregate data from THESIS and explore demographic, work-related and geo-economic characteristics of specialists treating hypothyroidism in Europe.

The THESIS survey was conducted according to the checklist for reporting results of Internet-based e-surveys (CHERRIES) (31). The target population consisted of members of national endocrine and/or thyroid related professional organisations who treated patients with hypothyroidism and comprised primarily endocrinologists, nuclear medicine physicians, and internists. This was a convenience sample (participants were selected through “convenient” data sources for researchers). Project oversight was provided by a Steering Committee (LH, EVN, EP, PP, RA and RN).

Thyroid specialists play a key role in promoting public health by facilitating early diagnosis and treatment, educating patients on the importance of managing their condition, and collaborating with other healthcare providers to manage and control their patients’ hypothyroidism and other related health conditions. Thyroid specialists are also involved in research and development of guidelines. By addressing these public health aspects, thyroid specialists can improve the overall health and well-being of individuals and communities affected by hypothyroidism.

We surveyed thyroid specialists belonging to national endocrine and thyroid professional organisations of the most populous European countries to document the demographic and work-related characteristics of physicians who treat hypothyroid patients in different European countries and explore associations with geo-economic factors. The overall valid response rate of 33% is comparable to response rates achieved by European physician surveys related to topics focusing on benign thyroid disease since 2008 (median 38%, range 26-52%) (40–43). Comparing physicians’ characteristics (sex distribution by country) from Eurostat (39) with THESIS showed negligible differences (the largest Cramer’s v was 0·027). It has been suggested that the relationship between the survey response rate and non-response bias is small (44). Despite this, we assessed non-response bias using two different approaches (34, 45). One was to assess the influence of measured variables on response rate, and another was to compare the distribution of sample variables with the distribution of the same variables in the reference population. Neither of these approaches showed systematic bias, suggesting that the survey reflects a representative sample.

Moreover, the response rates recorded in THESIS are likely to be underestimated, as national endocrine and thyroid professional organisations include members who are not clinically active and are most likely not to have responded to the questionnaire. The participants are likely to represent European specialists who manage thyroid diseases. The response rate was not statistically dependent on the language used in the questionnaire (national or English language), age, sex of the respondent, allowed time for responding or GNI per capita, but did vary by country, being lowest in the Netherlands, Switzerland, and Austria (6·8-19%) and highest in Germany, Ireland, and Bulgaria (79·1-95·2%). Several reasons could explain the national differences in response rates. In many European countries, patients with hypothyroidism are treated preferably by general practitioners whereas thyroid patients with more complex pathologies are referred to an endocrinologist. Also, endocrine societies include members who do not treat thyroid patients (pathologists, basic scientists etc). Furthermore, thyroid specialists from some national professional organisations are more accustomed to completing surveys (for example the Italian Associazione Medici Endocrinologi) and achieve high response rates compared to others (46). Response rates may also have been negatively influenced by the availability of potential THESIS participants involved with the ongoing COVID-19 pandemic.

An important question is whether the data reported here are relevant to the bulk of care delivered to hypothyroid patients at large. Complementary surveys targeting primary care physicians will be required to address this. However, the participating thyroid specialists (which included national opinion leaders) are likely to exert a major influence on practices in primary care at the national level. Furthermore, it is anticipated that the population of hypothyroid patients managed by thyroid specialists represents those patients who experience difficulties in achieving therapeutic targets or have persistent symptoms and utilise services most. The latter subgroup of hypothyroid patients has been the subject of intense research in the past three decades. Being aware of variations in demographic and geo-economic parameters of health providers is of interest (5).

Most respondents were female (65%), although this varied widely across countries (39-93%), with the largest differences observed between Northern and Eastern Europe, the former showing the lower proportion of female endocrinologists. Restricting the analysis to respondents younger than 60 years (to overcome the potential bias due to the difference in retirement age for men and women) did not significantly change the results. The increase in the proportion of women physicians is a universal finding. Eurostat data show that more females are enrolled in medical schools than men, reaching 60% in 2021 (47). Low salaries may disincentivise male workers and instigate their relocation to other higher-paying employment or countries with a higher GNI per capita. Other possible explanations include increased educational opportunities for women, changing gender norms and attitudes, and supportive policies and initiatives to promote gender equality. However, the influence of the GNI per capita favours the former explanation.

Hypothyroid patients seem to be managed predominantly by young female thyroid specialists in Europe, as demonstrated by the significant inverse linear relationship between the proportion of female respondents and age. This tendency applies across Europe as the relationship was present and robust in all the participating regions and is supported by the Eurostat data. This accords with the global increase in the proportion of women in all medical fields (48). Our finding of a female preponderance, particularly in Eastern European countries, is consistent with a recent trend of rising female sex ratios among Eastern European University staff (49). It has been speculated that low salaries disincentive male workers and instigate their relocation to other higher-paying employments or countries with a higher GNI per capita (49). Our finding of an inverse relation between GNI per capita and the proportion of female respondents is consistent with this hypothesis.

THESIS has shown that the work environment of thyroid specialists differs between countries. Private practice, in particular, varied widely. Of note, GNI per capita was inversely associated with the proportion of private practitioners. This may be explained by the fact that European countries with high GNI per capita have more efficient and easily accessible national healthcare systems than less affluent countries, resulting in a less needed private health sector. Higher salaries for thyroid specialists employed by the public health system in wealthy countries also may reduce the drive to work privately. The linear relationship between age and proportion of respondents engaged in private practice probably reflects a switch by the more experienced (and hence older) physicians from public to private practice in their later careers. Finally, the number of treated patients per year differed between countries and was inversely associated with the GNI per capita. This, too, is likely related to wealthy countries having a well-established and efficient primary care tier that manages most patients with hypothyroidism. The strong association between thyroid specialist characteristics and GNI per capita raises the important question of whether hypothyroid patients are treated differently according to geo-economic factors, and merits further investigation.

Strengths of THESIS include a large number of responses from nearly all European nations with a population of more than 4 million, utilisation of reliable channels for dissemination of the questionnaire in the form of national endocrine and thyroid professional organisations, exploration of a topic that has never been studied in such detail before and the potential repercussions of thyroid specialist characteristics on the patient experience.

There are also limitations: the questionnaire was not validated; the questionnaire had to be translated into several languages; the time for responding was unstandardised and varied substantially between nations; some of the national data were published and available before the end of the study, thus potentially allowing some of the countries access to how others in other countries had responded; the response rates varied considerably between countries; the impact of the COVID-19 pandemic (50, 51). To mitigate bias, strict inclusion criteria were applied, and robust statistical analyses were employed. However, the shortcomings alluded to above may have influenced the results of THESIS.

Thyroid specialists in Europe are predominantly young female endocrinologists, most of whom work in the public sector. Striking inverse associations were shown between the female sex and GNI per capita and private practice and GNI per capita. The most significant contributor to national differences was GNI per capita, but organisational aspects of each healthcare system, historical influences, and socio-political factors might play a role. Demographic and geo-economic variations among specialist healthcare providers for patients with hypothyroidism in Europe are notable and may have a bearing on patient-reported outcomes.

The datasets presented in this article are not readily available because these are data from members of 28 European Endocrine Societies, currently forming the basis of a multitude of national as well as aggregate data publications. Requests to access the datasets should be directed to milos.zarkovic@med.bg.ac.rs and laszlo.hegedus@rsyd.dk.

LH, EP, EN and PP conceptualized the study. MZ drafted the manuscript and performed the data analyses. MZ, LH EP, EN, RN, RA and PP participated in the interpretation of data, critically reviewed the manuscript, and approved the final draft. CAC, EA, MA, GA, TB, EB, MB, AB, MB, CB, MB, JC, JD, HD, VF, BF, EF, CF, DS, JG, TH, JJ, PK, MK, MK, MK, ML, IL, LL, VL, AM, MM, SM, CM, TM, TM, BB, DN, BP, TP, CP, ER, PR, MR, KR, AS, JF, DU, IV, WV, MV, YY, EY participated in data collection, critically reviewed the manuscript and approved the final draft. All authors contributed to the article and approved the submitted version.