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

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, 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?

• You're assuming 100% of the absorbed levothyroxine ends up in the bloodstream. I don't think that's a valid assumption. Aug 3, 2017 at 18:09
• It is important to note that people tend to have different responses to thyroid hormone supplementation, requiring different quantities depending on multiple factors, so I'm not sure there's a reliable correlation between dosage and serum T4 measurements. Aug 4, 2017 at 6:57