Abstract #1406483: Thyroid Hormone Resistance Diagnostic and Therapeutic Challenges

Abstract

Resistance to thyroid hormone (RTH) is a clinical syndrome defined by impaired sensitivity to thyroid hormone (TH). We present the challenges in diagnosing and treating a 24-year-old male who presented with a mixed picture of hypothyroidism and hyperthyroidism with elevated TSH, upper normal free T4, and elevated free T3 and was found to have a mutation in the THRβ gene. A 24-year-old male was referred for a second opinion of post-ablative hypothyroidism with elevated TSH, elevated free T3, and suspicion of pituitary microadenoma. He complained of fatigue, weight gain, tremors heat cold intolerance. Family history is Graves’ disease in his mother's status post partial thyroidectomy due to a salivary tumor. Initially, in 2020, he was diagnosed with negative antibody Grave’s disease. Labs were TSH 0.86 mIU/L, free T4 1.99 ng/dl (0.89-1.76), and free T3 4.66 pg/ml (2.3-4.2). A thyroid ultrasound showed mild thyromegaly. A thyroid uptake scan showed 24-hour thyroid uptake of 36.7%. Thyroid antibodies including thyroglobulin, TPO, thyroid receptor, and thyroid stimulating immunoglobulin were negative. He was treated with radioactive iodine ablation (I-131). Afterward, His TSH increased gradually along with free T3. Free T4 was normal. The patient remained symptomatic, and he was not on any thyroid replacement therapy. A repeat thyroid uptake scan showed an increase uptake of 36% at 24 hrs. During our evaluation, a repeat TSH 54.05 mIU/l, free T4 1.3 ng/dl, free T3 4.3 pg/ml, and T3 by equilibrium dialysis 4.0. A repeat magnetic resonance imaging of the head showed no definitive focal enhancing lesion. Pituitary axis hormones were normal, and the alpha subunit was 0.73 (high); however, the alpha-glycoprotein subunit /TSH molar ratio 0.135 sex hormone binding globulin 5.8 nmol/l (low), macro TSH was not detected. These findings raised suspicion for THRB mutation; hence genetic studies were obtained. PCR amplification of THRB gene exons 10, 9, and 8, followed by gene sequencing, showed a heterozygous missense mutation C >G located at exon 10. The mutation caused a change in thyroid hormone receptor beta protein amino acid 453 proline to an alanine, P453A Initially, he was started on levothyroxine 25 mg PO daily but soon after, it was stopped due to palpitations; a beta blocker was started along with liothyronine 5 mg every other day and levothyroxine 50 mg every other day. The patient tolerated the regimen with significant improvement in his symptoms. RTH is rather uncommon; it is important to be able to recognize the syndrome to avoid unnecessary invasive treatment. There is no specific therapy currently available to fully correct the TRβ defect. The goal of treatment is based on the patient's symptoms and clinical picture instead of aiming to normalize thyroid hormone levels. Thyroidectomy and radioactive iodine ablation result in lifelong levothyroxine replacement therapy and RTHβ persistently high serum TSH. Levothyroxine therapy is challenging, and supraphysiologic doses are often needed to maintain serum TSH in the lowest tolerable level.