Prevalence, incidence and associated comorbidities of treated hypothyroidism: an update from a European population.

OBJECTIVE: Estimates of the prevalence of hypothyroidism in unselected populations date from the late 1990s. We present an update on the prevalence and incidence of overt hypothyroidism in Piedmont, northwest Italy and examine the association between hypothyroidism and multiple chronic comorbidities.

DESIGN AND METHODS: Data were obtained from drug prescription and hospital discharge databases. Individuals who had received at least two levothyroxine prescriptions in 2012 were defined as having hypothyroidism; those who had undergone thyroidectomy or I131 irradiation in the previous 5 years were defined as having iatrogenic hypothyroidism and those who had either obtained exemption from treatment co-payment or had been discharged from hospital with a chronic comorbidity (diabetes and connective tissue diseases) were identified as having one of these conditions.

RESULTS: The overall crude prevalence was 31.1/1000 (2.3/1000 for iatrogenic hypothyroidism) and the overall crude incidence was 7/1000. The average daily dose of thyroxine (122 µg) roughly corresponded to 1.7 µg/kg. There was a strong association between hypothyroidism and diabetes (type 1, type 2 or gestational) and with autoimmune diseases, with the odds ratio ranging from 1.43 (1.02-1.99) for psoriatic arthritis to 4.99 (3.06-8.15) for lupus erythematosus.

CONCLUSIONS: As compared with previous estimates, the prevalence of hypothyroidism rose by about 35%, driven mainly by non-iatrogenic forms. The increase may be due to either population aging or improved diagnostic capability or both. The frequent co-occurrence of hypothyroidism with other multiple chronic conditions characterizes it more as a comorbidity rather than an isolated chronic disease.