Female patient (42 yr) suffered from autoimmune thyroiditis resulting in severe hypothyroidism. She was treated for several years by district physician with the dose of 150 microg L-thyroxine daily. Since the level of TSH was repeatedly very high and no improvement of clinical signs has been observed, she was referred to the Medical Faculty Hospital. Thyroid ultrasound showed remarkable diffuse hypoechogenicity, thyroid scintigraphy showed enlarged thyroid with low 99mTc uptake, TRH test was normal, thin needle biopsy supported autoimmune thyroiditis. X-ray examination showed normal sella turcica and no changes in the pituitary were observed with computer tomography. In spite of increasing the dose of peroral L-thyroxine to 300 microg/d and later to 500 microg/d the clinical status and TSH level did not improve. The patient was originally suspected from malabsorption of thyroxine. However, the test with a large single peroral dose (1000 microg) of L-thyroxine showed a rapid decrease of TSH level (from 126 to 75 mU/l) and increase of total T4 level (from 18 to 64 nmol/l) within 4 hr. Later the patient has been treated with intravenous L-thyroxine (500 microg every 3-4 days for 4 weeks) which resulted in the decrease of TSH level to 10 mU/l and increase of T4 level to 80-100 nmol/l. After that it was concluded that the problem is a poor compliance of the patient who apparently does not actually take the medication, although she always claimed that she is doing so. Referring to some similar cases described in the literature the case was classified as thyroxine pseudomalabsorption. In spite that this problem has been explained to her and her relatives, she refused to take any medication and is consistently neglecting all invitations to further examinations.