MRI-DEMONSTRABLE REGRESSION OF A PITUITARY MASS IN A CASE OF PRIMARY HYPOTHYROIDISM AFTER A WEEK OF ACUTE THYROID-HORMONE THERAPY

Although magnetic resonance imaging (MRI) characteristics of pituitary gland hyperplasia in primary hypothyroidism have been previously desc ribed, the time span necessary for the regression of the hyperplasia i n response to acute thyroid hormone (TK) therapy has not been defined. A 26-yr-old woman underwent I-131 ablation 11 yr before admission. In termittent poor compliance to levothyroxine (LT4) therapy led to inapp ropriately high serum thyroid-stimulating hormone (TSH) for her triiod othyronine (T3) and thyroxine (T4) levels. The patient was investigate d to rule out TSH-secreting pituitary adenoma or resistance to TH. On admission, the patient's clinical features and thyroid function tests, as well. as thyrotropin-releasing hormone (TRH) and acute T3 suppress ion tests, were in favor of profound primary hypothyroidism. MRI revea led symmetrical enlargement of the pituitary gland with distinct morph ological characteristics of a macroadenoma. The patient began high-dos e TH therapy and was rescannned six days later. The follow-up scan rev ealed a dramatic shrinkage of the pituitary gland. Four weeks later, s erum T4 and TSH were within the normal range, and repeat MRI scan of t he pituitary at that time showed a normal gland. This case is the firs t to document dramatic shrinkage of pituitary hyperplasia in long-stan ding primary hypothyroidism within one week of acute TH therapy. MRI a lone is unable to reliably differentiate between a TSH-secreting pitui tary adenoma and hypothyroidism-induced pituitary hyperplasia. Dynamic endocrine testing as well as repeat pituitary MRI after a brief TH tr ial may provide a firm diagnosis in similar cases.