Transition from Hashimoto thyroiditis to Graves's Disease: an unpredictable change?

Hashimoto thyroiditis and Graves's disease are two related autoimmune disorders, representing the leading causes of hypothyroidism and hyperthyroidism. Autoimmune hypothyroidism is generally irreversible but very rarely, some patients would shift to hyperthyroidism. The aim of the study was to seek for possible clinical predictors of the transition from hypo to hyperthyroidism in patients with Hashimoto thyroiditis and to outline their clinical phenotype.

Twelve patients with overt autoimmune hypothyroidism who had at least one transition from hypothyroidism to autoimmune hyperthyroidism were compared with 294 consecutive patients with autoimmune hypothyroidism and 69 consecutive patients with autoimmune hyperthyroidism that accessed the outpatient clinic over six months. Demographic, hormonal data and autoantibodies titers were compared.

Prevalence of smoking habit was significantly higher in switchers compared to controls. Switchers showed a significantly higher prevalence of personal and familial history of non-thyroidal autoimmune disorders. TSH levels were significantly lower in the switcher group during the hypothyroid phase and levothyroxine dose required was lower. TSH concentrations were significantly lower while free fT4 and free fT3 values were higher in GD patients compared to switchers during the hyperthyroid phase despite comparable TRAb levels. Prevalence and type of hyperthyroid symptoms and orbitopathy were similar between switchers and GD group. Mean dose of anti-thyroid drugs was significantly higher in GD patients compared to switchers. No differences were observed in the remission rate from hyperthyroidism between the two groups, despite switchers showed a significantly lower time-to-remission.

Conversion of Hashimoto Thyroiditis towards Graves' disease is a rare phenomenon which can occur almost at any time after the development of autoimmune hypothyroidism. Our findings suggest active surveillance of hypothyroid patients who require frequent reduction of levothyroxine during follow up and testing for TSHR antibodies in these patients.