We report pituitary enlargement after radioiodine ablation in a patient with elevated thyroid hormones and features of hyperthyroidism. Serum thyrotropin (TSH) levels were elevated despite normal circulating thyroid hormones, suggesting inappropriate TSH secretion associated either with a TSH secreting pituitary adenoma or resistance to thyroid hormone (RTH). Normal serum glycoprotein alpha-subunit levels and a preserved TSH response to thyrotropin-releasing hormone (TRH) favored RTH and this diagnosis was confirmed by showing the patient to be heterozygous for a missense mutation (R438H) in the thyroid hormone beta receptor (TRbeta) gene. Thyroxine replacement in supraphysiological doses were 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 replacement to prevent pituitary hyperplasia.