In quest of oral thyroxine resistance – three case reports with assumed intestinal malabsorption of levothyroxine

Oral substitution of thyroid hormone is the most common hormone replacement world-wide, but some patients fail to reach euthyroidism despite an elevated levothyroxine (LT4) dosage. If other causes are excluded, oral thyroxine resistance may be assumed which proposes a deficient intestinal uptake of LT4. We present three patients with abnormally high LT4 requirements. In case 1, an 18 year old hypothyroid female with Hashimoto's thyroiditis claimed a daily intake of 600 µg LT4. During thyroxine resorption testing, complete uptake of the administered LT4 dose was confirmed. Furthermore, test evaluation uncovered, that our patient must have taken about 600 µg LT4 by herself at the start of testing. When asked about this, she admitted non-compliance in the past. Currently, she receives a LT4-replacement adapted to body weight. In case 2, a 72 year old male patient had undergone surgical removal of benign goiter in 2012. He presented with a TSH of 2,3 mU/l under 400 µg LT4. Type B gastritis was diagnosed by gastroscopy and antibiotic treatment was initiated. Thyroid metabolism will be re-examined after gastric disease is cured. In case 3, a 57 year old female patient presented with 24 hours subcutaneous administration of LT4 filled in a morphine pump. This route of application was chosen because oral supplementation had failed after thyroid gland removal because of papillary cancer in 2001. Resorption testing revealed a four hours delayed uptake of ingested LT4. Moreover, abnormal aminoaciduria was diagnosed and could reflect a deficient transporter molecule localized in kidney and gut. Intestinal tissue is now analyzed for abnormalities in thyroid hormone/amino acid transporters. In summary, the etiology behind thyroxine malabsorption is broad, and non-compliance as well as objectively abnormal kinetics of thyroxine uptake has to be taken into consideration.

This project is funded by DFG SPP 1629 FU356/8 – 1.