Introduction: THR-B gene mutations result in resistance of thyroid hormone metabolism in specific tissues of the body. The pituitary and hypothalamus being the most sensitive which results in abnormally elevated thyroid hormone and TSH levels due to abnormal signaling i.e inability of T4 to reduce the elevated TSH. In many cases this can mimic hyperthyroidism making diagnosis crucial to prevent inappropriate management.
Clinical case: 61 YO M presented due to abnormal TFT’s with minimal to no symptoms. Initial tests completed were notable for subclinical TSH levels but repeat testing showed elevated TSH 5.740 (ref range 0.400-4.00 mIU/L) Elevated FT4 2.02 (ref range 0.89-1.80 ug/dL), and elevated FT3 4.8 (ref range 2.3-4.2 ug/dL). Alpha subunit levels were low/normal 0.39 (ref range < 1.37 ng/mL). Thyroid antibodies (Anti-TPO- IGG) levels < 8 negative (ref range < 35 IU/ml). The patient ultimately underwent genetic testing that confirmed thyroid hormone resistance beta syndrome. The patient was prescribed levothyroxine 50mcg QD due to persistent hypothyroid symptoms and scheduled for routine lab work to monitor TFT’s and clinical symptoms.
Conclusion: This syndrome is a rare inherited autosomal dominant disease that leads to abnormally reduced response of thyroid hormone in peripheral tissues. TSH levels can be normal or notably increased despite elevated TH levels in the blood. In simple terms the THRB gene is the receptor on many tissues of the body that allows the body to recognize thyroid hormone. THRB mutation leads to resistance of TH. The body will continue to increase levels of TSH and T4; these labs are often misdiagnosed as hyperthyroidism. Complications from this disorder are goiter development, a-fib, anxiety, osteoporosis, delayed growth and cognitive delay. This condition is treated with levothyroxine only when symptoms of hypothyroidism are present, or to suppress TSH to prevent goiter development. Routine TFT’s and visits are recommended for close monitoring. Recognition of THRB mutation is important to prevent misdiagnosis and avoid unnecessary antithyroid treatment.
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