Resistance to thyroid hormone--an incidental finding.

Area of Science:

• Endocrinology
• Genetics
• Molecular Biology

Background:

• Thyroid hormone resistance (RTH) is a rare genetic disorder characterized by elevated circulating thyroid hormones with a variable clinical phenotype.
• Distinguishing RTH from other thyroid conditions like Graves' disease is crucial for accurate diagnosis and management.

Observation:

• A 16-year-old female presented with mild hirsutism and a small goiter, along with a resting peripheral tremor.
• Initial investigations revealed elevated free thyroxine and total triiodothyronine levels, with a non-suppressed thyroid-stimulating hormone (TSH).
• Thyroid autoantibody testing and assessment for abnormal thyroid hormone binding proteins were negative, ruling out common causes of hyperthyroidism.

Findings:

• Radioiodine uptake scan suggested Graves' disease, but negative antibody tests and genetic analysis identified a thyroid hormone receptor gene mutation (T277I).
• This genetic finding confirmed the diagnosis of resistance to thyroid hormone (RTH).
• The patient exhibited elevated thyroid hormones with a lack of TSH suppression, consistent with RTH.

Implications:

• This case highlights the importance of considering RTH in the differential diagnosis of hyperthyroidism, especially when typical autoimmune markers are absent.
• Accurate genetic diagnosis of RTH is essential for appropriate patient counseling and management.
• Understanding RTH mechanisms can provide insights into thyroid hormone action and regulation.