A 30-year-old woman with a BMI of 31 and symptoms of hypothyroidism has a TSH of 35.6 mU/L. What must be determined for levothyroxine replacement calculation?

When calculating levothyroxine replacement doses for a patient with hypothyroidism, it is essential to use the ideal body weight rather than actual weight. This is because the dosage of levothyroxine is typically computed based on metabolic needs, which more closely align with ideal body weight due to the variable impact of excess body weight on metabolism.

Using ideal body weight ensures that the dosage reflects the patient's thyroid hormone requirements without the confounding effects of excess adipose tissue that may not contribute to thyroid hormone metabolism. Levothyroxine is dosed in micrograms according to body weight, and calculating based on ideal rather than actual weight helps to provide an appropriate dose for effective thyroid hormone replacement.

Other factors like BMI or TSH levels are important for overall assessment and management of the condition, but they do not directly inform the dosing calculation for levothyroxine replacement.