We report a case of severe hypothyroidism with nongoitrous, autoimmune thyroiditis and pituitary hyperplasia in a 13-year-old boy, who presented with sudden palsy on the left side of his face. Prednisolone and antiviral medication was administered. However, the facial palsy did not improve completely. The medications were replaced with thyroxine, and the facial palsy recovered. Endocrinological testing showed severe hypothyroidism as follows: thyroid stimulating hormone (TSH) level >100 µIU/mL, T4 of 1.04 µg/dL, T3 of 0.31 ng/mL, and free T4 of 0.07 ng/dL. Level of serum antithyroid peroxidase antibodies was 1,933.39 IU/mL, and that of antithyroglobulin antibodies was 848.16 IU/mL. Level of TSH receptor antibodies was >40 IU/L. Bioassay result for TSH receptor stimulating antibodies was negative. Thyroid sonography revealed no increase in the size or vascularity of the bilateral gland. Thyroid scintigraphy with 99mTc showed decreased uptake, and magnetic resonance imaging demonstrated an enlarged pituitary gland.