Differentiating Recurrent Thyroiditis From Graves’ Disease on a Background of Lithium Use

A 26-year-old female presented with severe thyrotoxic symptoms 4 months post-partum following an uncomplicated pregnancy; investigations showed a FT4 39.4 pmol/L (n 12 – 22 pmol/L), FT3 8 pmol/L (n 3.1 – 6.8 pmol/L), FT4:FT3 ratio 4.9, TSH <0.02 mU/L (n 0.27 – 4.2 mU/L), TSH receptor antibody (TRAb) < 0.9 IU/L (n < 0.9 IU/L) with normal blood flow on ultrasound doppler and a low (0.2%) Tc99 uptake isotope scan suggesting thyroiditis. 2 years previously she had an episode of thyroiditis (investigations showed FT4 31.2 pmol/L, FT3 7 pmol/L, FT4:FT3 ratio 4.4, TSH <0.02 mU/L and imaging showed low (0.4%) Tc99 uptake) when on lithium 1gm/day for more than a year to treat bipolar disorder. On recurrence, despite compliant treatment with Propranolol and Prednisolone 30mg once daily, thyroid function deteriorated and 2 months later repeat thyroid function testing showed a FT4 > 100 pmol/L, FT3 30.2 pmol/L, FT4:FT3 ratio 3.3 and TSH < 0.02 mU/L. A repeat Tc99 uptake scan showed increased uptake at 13% and ultrasound showed a very vascular bulky thyroid gland. These investigations were suggestive of either a rebound hyperthyroidism (post-thyroiditis) or Graves’ disease. At this stage, TRAb titers were repeated and were now elevated at 4.5 IU/L therefore carbimazole 40mg daily was started. Due to the severity of symptoms, she underwent urgent thyroidectomy. Histopathology showed features of diffuse hyperplasia of the thyroid gland with variably sized follicles lined by cuboidal epithelial cells and a mild patchy chronic inflammatory cell infiltrate in the stroma. Post thyroidectomy she is stable on L-thyroxine therapy. Points for discussion: 1. Lithium is typically used as a mood stabilizer and as thyroid dysfunction is known to exacerbate mood disturbances it is vital that patients are appropriately screened, monitored and treatment is commenced promptly for thyroid disease. 2. Lithium can increase clinical manifestations of thyroid autoimmunity and may influence thyrotoxicosis either due to thyroiditis or Graves’ disease. Differentiating these etiologies is important from the treatment perspective. 3. FT4:FT3 ratios, TSH receptor antibodies, Tc99 uptake scans and ultrasound doppler of the thyroid will assist in diagnostic accuracy. However, in certain rare circumstances, during the recovery phase of thyroiditis, the Tc99 scan can demonstrate diffusely increased activity, which is representative of a rebound phenomenon thus posing a diagnostic dilemma. 4. Finally, TRAb titers may help differentiate the above, but sometimes may change from undetectable to high suggesting changes in thyroid autoimmunity.