SAT-482 Simultaneous Hashimoto/Graves Disease or Prolonged Hashitoxicosis? A Diagnostic Challenge with Therapeutic Implications

Background: Some individuals with Hashimoto’s thyroiditis (HT), characterized by anti-TPO antibodies (Abs), can also have positive TSI Abs in up to 20% of cases, without necessarily having Graves disease (GD). Patients with signs of both hyper-and-hypothyroidism with positivity to these two Abs can pose a diagnostic and therapeutic dilemma, as their course is often unpredictable. Clinical Case: A 49-year-old woman was diagnosed with hypothyroidism and took levothyroxine (LT4) for about 1 year, after which she developed symptoms of hyperthyroidism and was switched to methimazole (MMI), which she only took for 1 year. At her initial visit at our clinic she had been off of MMI for 12 months and she was biochemically hyperthyroid (TSH of 0.01 mcIU/ml, f-T4 of 2.26 ng/dl). TSI Abs were positive at 461, but she also tested positive for anti-TPO at 673. Thyroid receptor Abs (TRAb) were also elevated at 54.8. Her vital signs were stable, but she had marked proptosis and complained of eye dryness, so MMI was restarted. A RAIU scan could not be obtained, but a thyroid US showed a heterogeneous and hypervascular gland. On MMI, her thyroid function tests normalized, and her eye disease vastly improved over 2 years. Her MMI dose was progressively decreased until it was stopped completely. On re-evaluation a few months later, she had newly elevated TSH of 8.7 mcIU/ml and low f-T4 of 0.87 ng/dl, with no symptoms of hypothyroidism, so we opted for management with active surveillance instead of starting her on LT4. Her TSI level improved to 240, but remains elevated. Discussion: It is unclear if our patient has a mixed condition with features of both GD and HT, or if she has HT with a very prolonged hyperthyroid phase (hashitoxicosis). Extended periods of hashitoxicosis have been described, the longest reported lasted for 2 years([1]). Simultaneous presentation of GD and HT is very rare, with only a few cases described in the literature. RAIU scan is often diagnostic, showing increased uptake as seen in GD, but patchy areas of decreased uptake can also be seen. In our case it is likely that HT and GD were coexisting, with GD masking the hypothyroidism, until the former remitted with MMI, and her HT took over. Though no RAIU scan was available, the TSI positivity, clinical response of her hyperthyroidism to MMI and the presence of orbitopathy rule in favor of co-existing GD. Decision to treat with LT4 should be weighed against the risk of causing recurrence of hyperthyroidism. Special considerations should be taken in women of childbearing age due to the difficult management that overlapping hyper/hypothyroidism would entail during pregnancy. References: 1. Shahbaz, A et al. Prolonged Duration of Hashitoxicosis in a Patient with Hashimoto’s Thyroiditis: A Case Report and Review of Literature. Cureus. 2018; 10(6):e2804