Pituitary Hyperplasia and Overt Hypothyroidism Induced by Methimazole in an Adolescent Girl with Resistance to Thyroid Hormone Accompanying Hashimoto’s Thyroiditis: A Case Report

We describe a 13-year-old girl who presented at her local hospital with a diffuse goiter and had discrepant thyroid function test (TFT) of elevated free T4 (FT4), free T3 (FT3) levels with mildly elevated thyroid-stimulating hormone (TSH) and a pituitary magnetic resonance imaging (MRI) report of a pituitary hyperplasia. She was referred to our hospital where a repeat TFT found low FT4 and high TSH levels, and high levels of antithyroid peroxidase (anti-TPO) and anti-thyroglobulin (anti-TG) antibodies, leading to the diagnosis of Hashimoto’s thyroiditis (HT) with overt primary hypothyroidism. The girl had a good response after daily 100 µg levothyroxine treatment for 8 months with decreased goiter size along with disappearance of the pituitary mass. However, her FT4 and FT3 levels were elevated while the TSH was in the high normal range, although at this time there were no signs of hyperthyroidism. A genetic study confirmed our provisional diagnosis that the patient had a p.Pro453Thr monoallelic loss-of-function mutation of the thyroid hormone receptor beta (THRB) gene, suggesting the diagnosis of coexisting resistance to thyroid hormone-β (RTHβ) and HT in this patient.