[LAHs]

There is a medication called Levothyroxine which some of us have to take if we have had a thyroidectomy (i.e. removal of thyroid gland as a result of cancer) or have a non functioning thyroid.
Background:
This medication consists of an organic compound called tetratiodothyronine or T4 for short. T4 because it has 4 iodine atoms attached to it’s rings, 2 to the outer phenolic benzene ring and 2 to the inner tyrosyl ring.
T4 is an inactive form and a very important process takes place where the body must knock off one of the iodine atoms (it’s done in the liver) in order to get to the active molecule called tri- iodothyronine  or (T3), T3 because this one only has 3 iodine molecules. T3 is the compound which gives the body energy.
My Question:
My question is, what sort of bond holds the iodine atoms to those benzene rings? Is it a Van de Waal bond? If not, does anyone know the name of that bond? And secondly, is such a bond variable in strength even in the very slightest. i.e. could one manufacture of T4 make a molecule where the iodine bond is too strong and makes it difficult for the body to knock off the iodine atom but another manufacturer makes a compound with a very slightly weaker bond, and thus easier to knock off?
Thank you,
LAHs

[mjc123]

It is a covalent bond. The bond strength is characteristic of the compound and cannot be variable between the same compound from different manufacturers.

[Babcock_Hall]

@OP, I agree with mjc123.  The only thing I would add is that the ability of an organism to break a chemical bond has a great deal to do with the enzymes in the body (enzymes are biological catalysts).  It has only a little to do with bond strength.

[LAHs]

I am trying to find out why SO many hypothyridic or post-thyroidectomy patients on the mono therapy of T4 only (levothyroxine or synthetic tetratiodthyronine – 4 iodine atoms) are so sick! 
T4 is administered orally via a pill. It is then up to the body’s chemistry to convert the T4 to T3, the hormone which will provide the energy for many other processes including the feeling of well being. T3 being Tri-iodothyronine – 3 iodine atoms.
It is often said that the patient is a feeble converter and that is why T3 is low or inadequate. This, of course may be the case for many patients but I would like to run a second theory passed you.
I have experience in mass spectrometry but with mostly elemental metals or simple compounds. We steered clear of long chain molecules since their rings would break up during analysis and produce a “forest of peaks” as we called it and often the peak of an impurity one might be looking for would coincide with a fragment of the original parent molecule.
My question is, do you  think that the manufactured synthetic levothyroxine molecule could break up before or when it hits the liver (where the de-iodination  takes place)? Therefore even if the patient is de-iodinating adequately with all the organic catalysts present, his source material has degenerated and therefore the end product is not T3 – hence low T3 and a feeling of malaise.
What do you think?

[Arkcon]

I hope you don't mind my merging your two posts.  They cover similar enough topics that its useful to have all the info together.

To start with, a gentler ionization technique can preserve large molecules for mass spec analysis.  Just an FYI for you, you comment on that topic seems to come from nowhere.  Unless you've tried to determine your wanted product in human serum.

I'd assume the metabolism of T4 to T3 is adequately documented in peer reviewed literature.  But if you can't find it, then yes the onus may be on you to verify the effectiveness of this therapy.

[Irlanur]

I have experience in mass spectrometry but with mostly elemental metals or simple compounds. We steered clear of long chain molecules since their rings would break up during analysis and produce a “forest of peaks” as we called it and often the peak of an impurity one might be looking for would coincide with a fragment of the original parent molecule.

The conditions in an MS are a "little bit" harsher than our usual environment and our body...

[Yggdrasil]

My question is, do you  think that the manufactured synthetic levothyroxine molecule could break up before or when it hits the liver (where the de-iodination  takes place)? Therefore even if the patient is de-iodinating adequately with all the organic catalysts present, his source material has degenerated and therefore the end product is not T3 – hence low T3 and a feeling of malaise.

The field of pharmacokinetics deals with studying what happens to drugs after we administer them to people, and the pharmacokinetics of almost all drugs that are in use should be known from testing done during clinical trials.  Because you are interested in whether T4 gets degraded, you're interested in the half-life of the drug, the time taken for the concentration of the drug to drop by half after administration (either through excretion of the drug or degradation of the drug in the body).

In the case of the pharmacokinetics of T4 it's halflife is reported to be quite long, in the 1-2 weeks range.  Thus, degradation of T4 occurs over the course of weeks.

[DrCMS]

@ LAH   A molecule of T4 from one source is chemically the same as another T4 molecule from a different source.
Your two theories are therefore false; the differences in energy levels etc are down to how those people metabolise T4 to T3 and further metabolise the T3 it is not caused by "differences" in the T4.