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Objective. The non-effectiveness of levothyroxine administration in hypothyroidism depends on many factors and mechanisms influencing its absorption in small intestins or bounding of circulating hormone with different active molecules. Methods. Thyroid hormones, TSH, rT3, TGl, TPOAb, TG-Ab, were measured using commercially available assays. For anti-T4ab, radioiodine-labeled T4 was added to the patient's serum and the IgG fraction subsequently precipitated by addition of 15% polyethylene glycol. Background was determined by testing 100 control sera from individuals without autoimmune thyroid disease. Results. A 42-year old woman (71.5 kg) with Hashimoto thyroiditis receiving levothyroxine (L-T4) 150 µg and liothyronine (L-T3) 37.5 µg was admitted to the hospital with clinical data of hypothyroidism, TSH-23.8 mU/L, FT4-6.18 pmol/L (n.range 9-19 pmol/L), TPO-Ab 696 IU/mL, TGAb 818 IU/mL, circulating T4-antibodies positive. She has a good adherence to medication, malabsorption or administration of other drugs were excluded. L-T4 absorption test revealed 44% increase of serum FT4 at 120 min after ingestion of 150 mcg L-T4 (2.1 mcg/kg). Methylprednisolone pulses of 500 mg i.v. administered in three consecutive days at equal doses of L-T4/L-T3 resulted in a rapid increase of FT4 to 14.5 pmol/L, fall of TSH to 0.18 mU/L and decrease of anti-T4 antibodies to referent range; TPO-Ab and TG-Ab also decreased significantly. Monotherapy by 150 mcg L-T4 was continued in the next three months. A recurrence of hypothyroidism with increase of circulating T4-Ab was observed 100 days later. New administrations of methylprednisolone two pulses of 500 mg revealed a similar normalization of thyroid hormones and anti-T4 antibodies. Conclusion. The data showed that T4-antibodies might be a cause of insufficient effects of levothyroxine therapy in autoimmune hypothyroidism. This could be overcome by glucocorticoid administration probably resulting in FT4 release from circulating immune complexes. [ABSTRACT FROM AUTHOR]