# Thread: modelling drug in body

1. ## modelling drug in body

The problem is: 130 micrograms of thyroxine is needed in her bloodstream. Her doctor prescribed one 100 microgram ,thyroxine tablet to be taken every day. If the half-time of thyroxine is 6.7 days, either justify that the uptake of thyroxine by the body and clearance through the liver fits a model similar to that for renal clearance stated above, or modify the mathematical model (do not alter the dosage or prescribe other drugs) so that it fits the given conditions.

I know already that to model the renal clearance, it would be a geometric progression with each term having exponential decay:
so
amount in body = De^(-0.10nT) + De^(-0.10 (n-1)T) +De^(-0.10 (n-2)T) +......+ D
because k = -0.10 So this is a geometric progression, defining variables: T = time interval between intakes, D = initial dose, n = nth number of dosage
But I know it is not as simple as this becasue this only modesl the renal clearance and not the absorption of the drug by the body. Could you please help me out? I don't know what I can do to the model to make it fit those numbers (100 per dose , resulting in 130 all the time in the body)

I also don't understand how you can find the long term amount of drug in the body using the initial dosages using a spreadsheet. If possible could you explain to me how you set out the spreadsheet? Becasue what I did to find the long-term saturation level was to take the sum to inifinity of the terms of that geometric progression.

This problem is quite urgentso appreciate ANY feedback!! Could yopu please help me out with the developemtns of the model to fit those numbers?

-thank you kindly!!!!

2. You cannot build a decent model with the given information. Still required are:

1) Time to maximum serum concentration, and
2) Absorption effectiveness.

I could not fine the time to maximum serum concnetration right off, but the absorption effectiveness of synthroid is an astoundingly broad 40%-80%! Maybe I'm just not looking, but that strikes me as a massively broad range. No wonder recommended administration requires a slow start. One would not want to assume 40% if you're going to get 80%.

Having said that:

Change in Concentration = Dosage*Effectiveness*(Function of Absorption Rate)*(Function of Clearance Rate)

So far, you have two of the four pieces for a basic model.

3. Originally Posted by linyen416