Challenging diagnosis of resistance to thyroid hormone in a patient with COVID-19, pituitary microadenoma and unusual response to octreotide long-acting release test

The resistance to thyroid hormone syndrome (RTHβ) occurs uncommonly and requires a high level of clinical suspicion and specific investigations to reach a precise diagnosis and to avoid unnecessary and potentially harmful therapies. We report a case of a young male patient referred to our unit for SARS-CoV-2 infection and atrial fibrillation with elevated thyroid hormones and non-suppressed thyroid-stimulating hormone (TSH), for which antithyroid therapy was prescribed. A mood disorder was reported in the medical history. The family history was unknown as the patient was adopted. Thyroid-specific antibodies were undetectable, and thyroid ultrasound revealed a normal thyroid gland without nodules. After the resolution of SARS-CoV-2 infection, the diagnostic workup continued, and the pituitary MRI revealed a small area ascribable to a microadenoma. Due to atrial fibrillation, the execution of the T3 test was contraindicated. The octreotide long-acting release (LAR) test showed an initial reduction of free thyroid hormones levels at first administration, which was consistent with the presence of a TSH-secreting pituitary tumour, although an escape from the response was observed after the following two injections of octreotide LAR. Indeed, the genetic investigation revealed a variant in heterozygosity of the THRβ gene (Pro453Ser), thus leading to an RTHβ diagnosis, and, therefore, medical treatment with triiodothyroacetic acid was initiated. After 2 years from the SARS-CoV-2 infection, the patient continues the follow-up at our outpatient clinic, and no other medical interventions are needed.