Type 2 diabetes mellitus and hypothyroidism: the possible influence of metformin therapy

Aims To evaluate the frequency with which hypothyroidism is associated with Type 2 diabetes, to examine gender and ethnic group differences, and to assess the possible impact of metformin therapy. To compare the prevalence of hypothyroidism in a cohort of people with Type 2 diabetes with a previously published cohort of people with Type 1 diabetes from the same centre. Method We randomly surveyed the records of 922 people with Type 2 diabetes (576 men and 342 women) to identify diagnoses of hypothyroidism, based on current thyroxin replacement therapy (with previous biochemical confirmation). Four subjects had secondary hypothyroidism after radio-iodine therapy for primary hyperthyroidism and were excluded from the analysis. The prevalence of primary hypothyroidism was documented in the remaining 918 subjects. We assessed the association of metformin therapy with hypothyroidism. Results The overall prevalence of primary hypothyroidism was 11.8% (women: 22.5%; men: 5.4%, P < 0.001) in subjects with Type 2 diabetes. Inter-ethnic differences were noted, with the highest prevalence among white subjects. The prevalence of hypothyroidism was lower in subjects with Type 2 diabetes who were receiving metformin therapy (P < 0.01), and this difference was greater when assessing those who developed primary hypothyroidism after starting metformin therapy (P < 0.001) Conclusions Our results support a relatively close association between diabetes and hypothyroidism. Hypothyroidism was more common in the cohort of white subjects than in other ethnic groups. The use of metformin therapy in people with Type 2 diabetes was associated with a significantly lower prevalence of diagnosed hypothyroidism.