SUN-277 Pituitary Hyperplasia Due to Severe Primary Hypothyroidism: An Uncommon Manifestation of a Common Disease

Background Pituitary hyperplasia (PH) is a non-neoplastic increase in one or more cell subtype of the adenohypophysis. It is physiologically seen in pregnancy from resultant lactotroph hyperplasia and pathologically in organ dysfunctions that lead to loss of negative feedback on hypothalamus like primary hypothyroidism and primary adrenal insufficiency. Although primary hypothyroidism is common, PH is an under-recognized consequence of long-standing hypothyroidism. Case Description A 26-year-old female with class 3 obesity, type 2 diabetes mellitus, and primary hypothyroidism on replacement presented for evaluation of secondary amenorrhea. Two months prior to presentation, she underwent a brain MRI for new-onset headache which revealed diffusely enhancing homogenous pituitary tissue measuring 2.3 cm with upward lifting of the optic chiasm. Serum prolactin was elevated at 86.2 ng/ml (2.8-26 ng/ml). This was presumed to be secondary to a prolactinoma and cabergoline was started by the referring provider. Subsequent workup revealed elevated thyroid-stimulating hormone (TSH) at 494.11 mU/L (0.30- 4.00 mU/L) and a low free thyroxine (FT4) of 0.2 ng/dl (0.8-1.7 ng/dl). Other pituitary hormones were within normal limits. The visual field examination was normal. It was noted she had a persistently elevated TSH over the past five years. On further questioning, the patient ran out of levothyroxine replacement 6 months ago and had been generally non-adherent to therapy for years due to nausea. Improved medication adherence resulted in the normalization of prolactin and reduction in TSH level. Repeat MRI 2 months later revealed a reduction in her pituitary hyperplasia to 2.1 cm. Discussion In severe primary hypothyroidism, there is loss of negative feedback to the hypothalamus due to low circulating FT4 and triiodothyronine (T3) concentrations. As a result, there is stimulation of thyrotropin-releasing hormone (TRH) which promotes thyrotroph hyperplasia. A strong correlation exists between TSH concentration and the degree of pituitary hyperplasia. The typical MRI finding is a diffusely enhancing homogenous pituitary mass. Despite optic chiasm contact, physiologic pituitary hyperplasia rarely causes visual field defects. High concentrations of TRH stimulates prolactin release with resultant mild hyperprolactinemia. Although PH is reversible with therapy, rare cases with subsequent development of panhypopituitarism while on therapy have been reported in the setting of longstanding hypothyroidism. Very rarely, chronically untreated primary hypothyroidism can stimulate the growth of a thyrotroph adenoma. Long-term biochemical and radiological monitoring is therefore recommended until resolution. This case highlights the physiologic responses manifested in severe primary hypothyroidism and the fact that these changes improve with adequate replacement.