Objective : Iodine is essential for thyroid hormone synthesis however most of Parenteral Nutrition (PN) formulas in USA does not contain added iodine. Patients on chronic PN are possibly at risk of iodine deficiency. This is a case of iodine deficiency causing hypothyroidism in an adult on chronic PN. A 19-year-old male patient with history of cerebral palsy, chronic respiratory failure on tracheostomy and short gut syndrome who was totally dependent on PN for at least 5 years. He had G-tube used for medications only. He presented with respiratory distress due to respiratory infection. CT neck was performed for tracheostomy assessment which showed heterogenous thyroid gland without goiter. This prompted thyroid function test which revealed hypothyroidism; TSH 95.49 mIU/L (0.4-5 mIU/L), free T4 <0.23 ng/dl (0.7-1.9 ng/dl). Antithyroid peroxidase antibody (anti-TPO) level was normal which raised the possibility of iodine deficiency as the cause of hypothyroidism. The PN the patient was receiving didn’t contain labeled iodine. Patient was treated with levothyroxine and Prenatal multivitamins daily which contains 150 ug of iodine per tablet. Levothyroxine was stopped 3 weeks later and hypothyroidism resolved as was seen on multiple thyroid function tests.
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Discussion : Iodine is not added to most PN formulas in USA due to the assumption that patients on PN receive iodine via other sources like skin absorption of iodinated antiseptics used for routine central line care and many of the patients on chronic PN are still able to have limited oral intake. Chlorhexidine has replaced iodinated antiseptics which raises a concern of iodine deficiency in patients on chronic PN. Goiter is one of the thyroid adaptive mechanisms to iodine deficiency and typically it precedes hypothyroidism. The patient did not have goiter on CT neck however his thyroid gland looked heterogenous. Thyroid Ultrasound was not performed due to technical difficulty in the presence of the tracheostomy. It would be ideal to obtain UIC to confirm iodine deficiency but this test was not obtained. The absence of anti-TPO antibodies and resolved hypothyroidism after few weeks of iodine replacement are strong indications for iodine deficiency. Patient was admitted several times for unrelated medical problems and he continued to be euthyroid while on iodine supplement.
Iodine is not added to most PN formulas available in USA and thus patients on chronic PN are possibly at risk of iodine deficiency and thyroid dysfunction. We suggest routine thyroid function test in patients on chronic PN. Further studies are needed to assess iodine status in patients on chronic PN.
Maha kishk– Endocrinology Fellow, University of Missouri in Kansas City, Olathe, Kansas