Thyroid Hormone Resistance Syndrome Presenting as Exercise Induced Asthma

Background: Pathogenic variants in the thyroid hormone receptor (THRB) gene are associated with thyroid hormone resistance. Over 85% of the genetic mutations are in the beta TR gene. The disorder is characterized mainly by elevated thyroid hormone levels, unsuppressed levels of TSH, and goiter. Clinical case: An 11-year-old male initially seen at the PCP office for tachycardia and chest discomfort, especially during school exercise. He reported shortness of breath. His symptoms will resolve at rest. He was diagnosed initially with asthma and was put on Singular, Pulmicort, and albuterol as needed. His symptoms did not improve with this treatment, and he reported symptoms worsened with albuterol. He had a normal Echocardiogram and chest X-ray. A mild goiter was noted on his physical exam. Thyroid ultrasound showed an enlarged thyroid gland. An ovoid echogenic focus in the inferior thyroid lobe measuring 4 X 7 X 8mm was identified. The nodule was wider than tall, with a solid appearance with no internal color flow. Evaluation in our clinic showed normal TSH at 1.624mcIU/mL (0.35-5.5). FT4 was high at 1.54ng/dL (0.82-1.40), and FT3 elevated at 6.3pg/mL (3.3-4.8). Thyroid antibodies and thyroid-stimulating immunoglobulin (TSI) were normal. A 24 hour I-123 thyroid uptake was approximately 55% (10-30%) with no focal increased or decreased uptake. Given his elevated thyroid hormone levels with unsuppressed TSH in the context of goiter and tachycardia, genetic testing for the Thyroid receptor gene was done. He was found to be heterozygous for a pathogenic variant in THRB, c.1286G>A (p.Arg429Gln). This genotype is consistent with a diagnosis of autosomal dominant Thyroid hormone resistance. The patient was started on Atenolol, given his elevated heart rate, and he reported improvement in his symptoms during exercise. Conclusion: Thyroid hormone resistance was first described as a clinical entity in 1967. The phenotype can vary among individuals. It is characterized by a reduced responsiveness of target tissue to thyroid hormone and binding affinity. The disease can present with goiter, behavioral issues, abnormal growth, and tachycardia. Affected individuals may have attention deficit-hyperactivity disorders (ADHD) and language difficulties. Thyroid hormone resistance can be misdiagnosed, as in our patient. He was diagnosed with asthma and was put on unnecessary medications that worsened his symptoms. Thyroid hormone resistance can also be misdiagnosed with Graves’ disease, given the elevated thyroid hormone. It is essential to highlight the importance of genetic testing in these cases, as an accurate diagnosis will prevent unnecessary treatments with potentially serious side effects.