REVERSIBLE PITUITARY ENLARGEMENT IN THE SYNDROME OF RESISTANCE TO THYROID-HORMONE

We report pituitary enlargement after radioiodine ablation in a patien t with elevated thyroid hormones and features of hyperthyroidism. Seru m thyrotropin (TSH) levels were elevated despite normal circulating th yroid hormones, suggesting inappropriate TSH secretion associated eith er with a TSH secreting pituitary adenoma or resistance to thyroid hor mone (RTH). Normal serum glycoprotein alpha-subunit levels and a prese rved TSH response to thyrotropin-releasing hormone (TRH) favored RTH a nd this diagnosis was confirmed by showing the patient to be heterozyg ous for a missense mutation (R438H) in the thyroid hormone beta recept or (TR beta) gene. Thyroxine replacement in supraphysiological doses w ere required to normalize TSH levels and resulted in regression of the pituitary enlargement, suggesting hyperplasia rather than coincident tumor. This case illustrates the need to avoid thyroid ablation in RTH patients and the importance of supraphysiological thyroxine replaceme nt to prevent pituitary hyperplasia.