Unusual Malabsorption of Levothyroxine

We report a 50-year-old woman, with overt hypothyroidism undergoing thyrotropin (TSH)-stimulating hormone suppressive levothyroxine (LT₄) treatment after subtotal thyroidectomy. At her first visit to our department, the laboratory results revealed a borderline low free thyroxine (FT₄) level accompanied by a clearly elevated TSH level. Both parameters did not significantly change during therapy with an oral dose of 500 μg of LT₄. Investigations revealed malabsorption of oral administrated LT₄. Thyroid serum hormone levels only became normal during parenteral therapy with LT₄.