Ms TM is a 21 years old Eurasian female presenting with goitre and TSH of 1.99 mIU/L (0.4-3.8), free T4(FT4) of 23.9 pmol/L (10-22) and free T3(FT3) of 6.6 pmol/L. She has one year’ history of fatigue, muscle weakness and decline in her academic performance. She has history of appendicetomy and family history of leukemia. Her height was 153 cm, weight 54.5 kg and BMI of 23.3 kg/m2. She has a non-tender, firm right thyroid nodule. Thyroid ultrasound revealed a normal sized thyroid gland with a mixed solid and cystic nodule measuring 29x21x15mm in the interpolar region. Repeated TFTs at different laboratories demonstrated persistent elevation of FT4 and FT3 and non-suppressed TSH (Table 1). Several assays confirmed the absence of heterophil antibodies. Her pituitary profile was normal. She has elevated an alpha subunit of 0.82 IU/L (0-0.6) and a normal sex hormone binding globulin (SHBG) of 48 nmol/L (20-118). Magnetic Resonance Imaging (MRI) of the pituitary revealed multiple small cystic lesions in the posterior aspect. Thyotropin (TRH) stimulation test showed exaggerated TSH response. She is currently awaiting thyroid hormone receptor beta (THRB) mutation testing.
Resistance to thyroid hormone (RTH) is characterised by reduced sensitivity of target organs to thyroid hormone1,2,3. Eight five percent of cases are caused by mutation in THRB which is inherited in autosomal dominant pattern while 28% occurred denovo.1 Incidence of RTH is reported to be one case per 40,000 live birth4. Gotire is present in 65-95% of the cases5. An important differential diagnosis is TSH-secreting pituitary adenoma2,5. In TSH-producing tumour, the ratio of alpha subunit-to-TSH will be raised. Management of RTH is aimed at managing the patient’s clinical symptoms2,6. Normalising of thyroid hormone levels is not required2,6. Management of RTH in pregnancy is challenging2,6. Family counselling and genetic testing are a crucial part of RTH management2,6.