Endocrine Abstracts

Introduction: Thyroid disorders are commonly encountered in clinical practice, but their presentations can often be subtle or misleading. Biochemical test results must always be interpreted in conjunction with the patient’s clinical picture to avoid misdiagnosis. In particular, differentiating between central hypothyroidism and thyroid hormone abnormalities caused by autonomous thyroid nodules can be challenging.

Case Presentation: We report the case of a 49-year-old lady referred for evaluation of a multinodular goitre. Ultrasound imaging identified benign thyroid nodules, and initial thyroid function tests revealed a slightly suppressed TSH with normal T3 and T4 levels. However, subsequent blood tests showed persistently low TSH and low T4, although T3 was not measured at that time. Based on these findings, she was diagnosed with secondary hypothyroidism and started on Levothyroxine therapy. When the Levothyroxine dose was increased to 100 mg, the patient developed symptoms consistent with thyrotoxicosis. Interestingly, she reported no significant difference in well-being whether on or off treatment and had no prior history of hypothyroid symptoms. Her obstetric history included two uneventful pregnancies. Further endocrine assessment revealed persistently suppressed TSH with elevated or high-normal free T3, and normal free T4 levels. A TRH stimulation test showed a non-responsive TSH, while pituitary MRI and other pituitary hormone levels were normal. Considering the clinical context and biochemical profile, a nuclear medicine thyroid uptake scan was performed, confirming T3 thyrotoxicosis due to a toxic thyroid nodule. Levothyroxine was discontinued to avoid worsening thyrotoxicosis.Following detailed discussion, the patient elected to undergo hemithyroidectomy as definitive treatment for the toxic nodule.

Conclusion: This case highlights the diagnostic challenge in distinguishing central hypothyroidism from TSH suppression caused by autonomous thyroid hormone secretion from a toxic thyroid nodule. Persistent elevation of FT3 combined with suppressed TSH and the absence of pituitary pathology should prompt consideration of toxic nodular disease.