How much T4 increase do you get per mcg of levothyroxine?

Question

A normal starting dose of levothyroxine is 50 mcg per day. According to ATA, absorption is 60-80%. Let's assume 70% and let's assume 5 L of blood in the average human being. 50 mcg * 0.7 / 5.0 = 7 mcg/L 7 mcg/L = 7000 ng/L According to [this tool](http://unitslab.com/node/121), 7000 ng/L of thyroxine is 9000 pmol/L. Free T4 is usually 0.03% of total T4 so I'd at first expect the dose to contribute `9000 * 0.03 = 270 pmol/L` of free T4. But a normal human range of free T4 is 10-24 pmol/L. Clearly I've made a mistake somewhere since the dose seems to be an order of magnitude too large. What's the correct calculation? What is the expected average T4 increase when taking 50 mcg of levothyroxine?

Answer 1

This is a hard question… and to be honest it varies based on a person’s height, weight, metabolism, other medications taking, if taken on an empty stomach versus with food, ect. I wouldn’t be able to give you an estimate of what 50 mcg would do to a hypothetical person, as I wouldn’t know their weight, height, activity level, Comorbidities, diet, activity level, compliance, ect. On average, a dose of 1.6 mcg per kg per day dosing results in a return of your TSH to a normal level or 0.5-5 and typically results in a T4 level that WNL (the range you listed) or … but it is not an exact science and levels are monitored every 8 weeks or so…for which the medication is then titrated up or down based on those results until thyroid labs are and continue to be WNL. It is different for everyone. And elderly typically need less than active young-middle age adults. Hopefully this helps.