Pituitary Hyperplasia Secondary to Primary Hypothyroidism Mimicking a Macroadenoma With Optical Chiasm Compression

Background: A long term untreated primary hypothyroidism can stimulate thyrotropes proliferation, leading to pituitary hyperplasia. This condition is known as pituitary hyperplasia secondary to primary hypothyroidism (PHPH). It is a rare condition that mimics pituitary adenoma and can achieve large proportions with optic chiasm compression. A misdiagnosis may be catastrophic, and a pituitary resection wrongly performed. Clinical Case: A 25-year-old woman with a medical history of delayed neuro psychomotor development and epilepsy due to congenital CNS malformation had a Brain MRI performed for epilepsy follow-up three years earlier. At that time, neuroimaging had shown a pituitary enlargement, and laboratory data were diagnostic of severe primary hypothyroidism with TSH of 290.6 uUI/mL (normal range 0.4 to 4.5 uUI/mL) and T4L <0.23 ng/dL (normal range: 0.9 to 1.8 ng/dL). She then had received a 75 mcg levothyroxine prescription. However, the patient missed medical follow-up and returned three years later, when sella turcica MRI showed a 0.9x1.0x1,4 cm pituitary lesion, consistent with a macroadenoma with suprasellar extent near the optic chiasm. Because of the cognitive impairment, the patient was not able to complete the visual field test. Neurology service referred the patient to endocrinology evaluation for surgical treatment. Hypothyroidism was still uncontrolled with TSH 157.1 uUI/mL and T4L 0.28 ng/dL. We had adjusted the levothyroxine dose to 125 mcg and advised adherence. Subsequent thyroid function tests had shown TSH 6.91 uUI/mL and T4L 1.15 ng/dL. After thyroid function stabilization, the patient performed a new sella turcica MRI, which had not evidenced pituitary lesion. Pituitary hyperplasia secondary to primary hypothyroidism was her final diagnosis. Conclusion: This case report illustrates the importance of the correct diagnosis and treatment of PHPH. Levothyroxine replacement, with TSH normalization, reverses the gland hyperplasia within 2 to 4 months. References: 1. Cao J, Lei T, Chen F, Zhang C, Ma C, Huang H. Primary hypothyroidism in a child leads to pituitary hyperplasia: A case report and literature review. Medicine (Baltimore). 2018 Oct;97(42):e12703.2. Shivaprasad KS, Siddardha K. Pituitary Hyperplasia from Primary Hypothyroidism. N Engl J Med. 2019 Feb 21;380(8):e9.