Summary statements

• Monitoring serum T3 levels and fT3/fT4 ratios on conventional LT3 preparations is problematic due to 24 h variation.

• Interpretation of TSH levels on conventional LT3 preparations may also be problematic.

• Physiological dosing of LT3 is difficult to achieve using conventional T3 preparations.

• Slow-release T3 preparations are required to achieve physiological fT3/fT4 levels.

• Slow-release T3 preparations are required to achieve physiological TSH levels in combination with physiological fT3/fT4 levels.