Use of T4, T4 + T3, and T3 in the Dutch Population in the Period 2005–2011

Abstract

As chairman of the Dutch Association of Graves' Patients, I was recently invited to speak at a symposium on personalized medicine. The organizers had asked me to present figures on the use of thyroid hormone (especially the combination of T4 and T3) in the Netherlands. With the help of the Royal Netherlands Society of Pharmacy (Koninklijke Nederlandse Maatschappij ter bevordering der Farmacie), we approached the Foundation Pharmaceutical Indicators (Stichting Farmaceutische Kengetallen). They were able to provide us with key figures on the use of thyroid hormone in the Netherlands over the period 2005–2011 (courtesy of F. Griens). The Foundation collected prescription data from 843 public pharmacies, which had not changed the unique patient identification numbers in the period 2005–2011. The pharmacies linked to the Foundation constitute a representative sample of all pharmacies in the country. The figures obtained from the 843 pharmacies were extrapolated to all Dutch pharmacies, serving the total population of the Netherlands. The number of persons using any thyroid hormone medication increased by 53% in the period 2005–2011; the proportion of the total Dutch population using thyroid hormone increased by 49%, whereas the Dutch population size in the same period increased by 2.1% (table ​(table1).1). Distinguishing between the various thyroid hormone preparations, the number of persons using only T4 increased by 53%, those using the combination of T4 + T3 increased by 67%, and those using T3 only increased by 36%. Among the persons using thyroid hormones, the proportion of only T4 users gradually but consistently decreased from 99.05% in 2005 to 98.98% in 2011. An opposite trend was observed in the T4 + T3 users, which slowly but steadily increased from 0.82% in 2005 to 0.90% in 2011. The proportion of only T3 users did not show a specific trend – it was 0.13% in 2005 and 0.12% in 2011. The assembled data also allowed to analyse whether prescriptions were made by general physicians (GPs) or specialists (table ​(table2).2). With regard to prescriptions for T4 there was a clear trend that in the period 2005–2011, T4 prescriptions by GPs became more frequent (from 79% of all T4 prescriptions in 2005 to 90% in 2011), whereas T4 prescriptions by specialists decreased (from 21 to 10%). The annual proportion of persons starting on T4 fluctuated between 19 and 21% of all thyroid hormone users. Initiation of T4 treatment by GPs increased (from 62% in 2005 to 71.5% in 2011) at the expense of initiation by specialists (from 38% in 2005 down to 28.5% in 2011). With regard to T3 prescriptions, a similar picture emerges. The proportion of T3 prescriptions by GPs rises from 52% in 2005 to 68% in 2011, under a simultaneous fall in T3 prescriptions by specialists from 48 to 32%. The annual proportion of persons starting T3 fluctuated between 0.38 and 0.49% of all thyroid hormone users. Initiation of T3 treatment was done by GPs in 37% in 2005, but had increased to 45.5% in 2011; the figures for specialists declined from 63 to 54.5%. Table 1 Thyroid hormone medication in the Dutch population in the period 2005–2011 Table 2 Prescriptions for T4 and T3 in the Dutch population in the period 2005–2011 in % Two conclusions can be drawn. The number of persons using thyroid hormone medication increased substantially in the period 2005–2011. And the proportion of thyroid hormone medication prescribed by GPs steadily increased in this period, whereas the proportion prescribed by specialists decreased. How can these remarkable trends be explained? We offer a few causal mechanisms. First, the growth of the Dutch population size between 2005 and 2011 will have contributed to the increased number of persons taking thyroid hormone. However, this provides only a partial explanation because the relative increase of thyroid hormone users exceeds by far the relative increase of population size. Second, the composition of the Dutch population is changing with a strong increase of the ageing population. As the incidence of Hashimoto's hypothyroidism increases with advancing age, a greying population will increase the use of thyroid hormones. Nevertheless, this cannot be the whole explanation because the effect of the ageing population is just three times greater than the effect of the general population growth. Third, the Dutch College of General Practitioners (Nederlands Huisartsen Genootschap) published in 2006 guidelines for the diagnosis and treatment of thyroid function disorders [1,2]. At the implementation of the guidelines, specific attention was drawn to the condition of subclinical hypothyroidism. It is plausible that the guidelines to a large extent have contributed to the increased use of thyroid hormone, also explaining the shift in prescription behaviour away from the specialists towards the GPs. In further support of this explanation is the introduction of specific software used by labo