Endocrine Abstracts

Introduction: Resistance to thyroid hormone (RTH) is a rare inherited syndrome characterised by refractoriness of target tissue to thyroid hormone. Over 80% of cases are due to mutations in the thyroid hormone receptor beta (THR β) gene with over 100 mutations identified to date. The clinical manifestations vary from common feature as goitre to less common sinus tachycardia, learning disabilities, growth and developmental delay.

Aim: We report the case of a 4-year-old boy with incidental pick up of abnormally elevated free thyroxine and normal TSH biochemistry with absence of goitre or thyrotoxicosis features.

Case Presentation: 4-year-old boy was referred to endocrine clinic following two-week history of lethargy, pallor, angular stomatitis. History suggested periods of hyperactivity, hand tremors, anxiety and poor weight gain with BMI 13.8 kg/m². There was no family history of thyroid disease, any safeguarding concerns or accidental ingestion. Investigations revealed normal full blood count, free thyroxine (fT4) of 86.6pmol/l (range 11-22) , fT3 of 23.9pmol/l (range 3.5 – 8.5) and TSH 2.59mu/l (0.7 – 6). Thyroid functions checked on alternate assay re-affirmed the same status. Heterophile antibody screen was negative. In contrast to the history, the child did not have clinical signs of hyperthyroidism or goitre. His heart rate, 24 hour ECG and Echocardiogram were normal. Chest x-ray showed normal cardiac and mediastinal contours. TSH Receptor Antibody (TRAb) was normal, making autoimmune thyroid disease less likely. A genetic panel screen for THR β gene mutation was considered which confirmed heterozygous THR beta mutation at Chr3:g.24164388A>G. This variant has been previously identified in only three additional patients with RTH. A genetic workup has been initiated for parents. He is asymptomatic without the need for any treatment at the 8-month follow-up.

Conclusion: Typically, children with THR β gene mutation have goitre and mild or no evidence of thyrotoxicosis. Our case demonstrates that one should consider RTH due to THR β when faced with biochemical findings of raised fT4, fT3 with normal TSH levels in the absence of clinical signs of thyrotoxicosis. Clinician’s awareness can avoid unnecessary treatment which is not required in most cases, as the elevated thyroid hormones levels compensate for the partial tissue resistance.