A Rare Case of a Toxic Thyroid Nodule Found in a Hypothyroid Patient on Levothyroxine Therapy

Introduction: A hyperfunctional thyroid nodule can lead to symptoms of overt or subclinical hyperthyroidism but the association between a hyperfunctional thyroid nodule and hypothyroidism has not been well reported. We present a patient with a prior history of hypothyroidism previously controlled on Levothyroxine who later presented with an enlarging hot nodule. Case Presentation: A 62-year-old female with a history of factor V Leiden, hypothyroidism on levothyroxine therapy, and a meningioma presented to an outpatient clinic with complaints of fatigue, constipation, and 37-pound weight loss in one year. She was diagnosed with hypothyroidism 7 years ago after delivering her third child, but the underlying cause of her disease was unknown. She began taking levothyroxine 50mcg every morning after her diagnosis. She reported compliance and proper pill taking technique. Physical examination revealed a palpable thyroid nodule. The patient had a previous work up for thyroid nodules with a thyroid uptake and scan a few years prior, which showed a 1.42 x 0.96 x 1.87 cm hot nodule at the right middle lobe with a 24-hour uptake of 15.3%. The patient was asymptomatic at that time and thyroid function tests were within normal limits. She was instructed to continue taking levothyroxine. Repeat RAI Uptake scan at the time of her presentation to our office again showed the right middle lobe hot thyroid nodule with an increased 24-hour uptake of 27.5%. Ultrasound showed bilateral thyroid nodules and a hypervascular solid nodule measuring 2.28 x 1.27 x 1.9 cm that has increased in size. Lab work at this visit revealed a TSH of 0.329 uIu/mL, and free T4 of 1.25 ng/dL. Due to her low TSH and clinical presentation, the levothyroxine was discontinued. Anti-thyroid peroxidase antibodies were obtained to assess for Hashimoto’s Thyroiditis but were found to be normal. The patient was later referred to an endocrine surgeon for a total thyroidectomy. Conclusion: Although uncommon, hyperfunctional nodules in hypothyroid patients can create a confusing clinical picture with overlapping symptoms of underactive and overactive thyroid disease. It has been reported that patients with Hashimoto’s Thyroiditis can have hot nodules and coexisting hypothyroidism but the prevalence of hyperfunctional nodules in hypothyroid patients without Hashimoto’s Thyroiditis, as in this case, is not well-documented. Patients with hypothyroidism are treated with Levothyroxine but if coexisting hyperfunctional nodules are not detected, the patient may develop thyrotoxicosis. Clinicians should be aware of this rare but potentially life-threatening clinical condition.