Thyroid hormones in infants undergoing cardiac surgery

Thyroid hormone has been tested during and after cardiac surgery with the hypothesis that it may enhance cardiac contractility of the uninjured or failing myocardium in situations where thyroid metabolism is impaired. There is a single trial of oral thyroxine in in infants undergoing cardiac surgery [30]. Talwar et al 2018 in infants undergoing open-heart surgery compared oral thyroxin 5 microgram/kg 12 hours before surgery and once daily for the remainder of ICU stay versus placebo. Oral thyroxin supplementation improves the cardiac index, reduced inotrope requirement, duration of mechanical ventilation, ICU and hospital stay and therapeutic intervention scoring system score.

Triiodothyronine (T3) is the biologically active hormone, but there is no evidence that combined therapy with levothyroxine and liothyronine is more beneficial than treatment withlevothyroxine alone, probably due to the high degree of efficiency of endogenous deiodinases which break T4 down into T3.[2] levothyroxine is available in tablet but not licensed as a liquid form in Australia. Liquid preparations may be better absorbed, particularly in patients with malabsorption and in newborn infants in whom lower TSH levels were reported.[31-31] Suspensions prepared by pharmacists may not allow reliable dosing.[2] Brand and generic levothyroxine are not bioequivalent so it is prudent to use a brand preparation, particularly in severe cases.[2, 34]

Peak concentrations occur 2 to 4 hours after oral administration. Therefore blood for thyroid function tests should preferably be taken immediately before a dose is due [1]. Fasting will increase the extent of absorption, whereas malabsorption may decrease absorption.

The daily levothyroxine tablet should be crushed and mixed with water, expressed breast milk, or formula. Although it is recommended to administer levothyroxine on an empty stomach and avoid food for 30–60 min, this is not practical in an infant. levothyroxine should not be mixed with substances that interfere with gastrointestinal absorption, such as soy protein formula, concentrated iron, or calcium.[16]

Levothyroxine sodium is variably but adequately absorbed from the gastrointestinal tract following oral administration. Approximately 50 to 80% of levothyroxine sodium is absorbed.[2] Elimination half-life is about 6–7 days.[35, 36]

Commencing thyroxine 10 to 15 microgram/kg/day will normalise serum free T4 or T4 in 3 days and TSH in 2 to 4 weeks following the initiation of therapy.[16]