Sorry - but the simple answer is that while it is an interesting question in relation to people with ME/CFS who may have might be termed 'borderline hypothyroidism' I don't have a reliable answer
We receive a lot of feedback from people with ME/CFS who are being investigated and treated for hypothyroidism
Some of them don't get the benefit from treatment that would be expected (for reasons which are sometimes difficult to understand) and some of them have problems with side-effects
The latter can, of course, be due to receiving a higher dose of thyroxine than they can cope with and also occurs when people with no other health problems are treated with thyroxine
Here is some basic UK info on management of hypothyroidism:
Treating an underactive thyroid
An underactive thyroid (hypothyroidism) is usually treated by taking daily hormone replacement tablets called levothyroxine.
Levothyroxine replaces the thyroxine hormone, which your thyroid doesn't make enough of.
You will initially have regular thyroid blood tests until the correct dose of levothyroxine is reached. This can take a little while to get right.
You may start on a low dose of levothyroxine, which may be increased gradually, depending on how your body responds.
Some people start to feel better soon after beginning treatment, while others don't notice an improvement in their symptoms for several months
Once you are taking the correct dose, you will usually have a blood test once a year to monitor your hormone levels
If blood tests suggest you may have an underactive thyroid, but you don't have any symptoms or they're very mild, you may not need any treatment.
In these cases, your GP will usually monitor your hormone levels every few months and prescribe levothyroxine if you develop symptoms.
Taking levothyroxine
If you are prescribed levothyroxine, you should take one tablet at the same time every day. It's usually recommended that you take the tablets in the morning, although some people prefer to take them at night.
The effectiveness of the tablets can be altered by other medications, supplements or foods, so they should be swallowed with water on an empty stomach, and you should avoid eating for 30 minutes afterwards.
If you forget to take a dose, take it as soon as you remember, if this is within a few hours of your usual time. If you don't remember until later than this, skip the dose and take the next dose at the usual time, unless advised otherwise by your doctor.
An underactive thyroid is a lifelong condition, so you'll usually need to take levothyroxine for the rest of your life.
If you're prescribed levothyroxine because you have an underactive thyroid, you're entitled to a medical exemption certificate. This means you don't have to pay for your prescriptions.
Side effects
Levothyroxine doesn't usually have any side effects, because the tablets simply replace a missing hormone.
Side effects usually only occur if you're taking too much levothyroxine. This can cause problems including sweating, chest pain, headaches, diarrhoea, and vomiting.
Tell your doctor if you develop new symptoms while taking levothyroxine. You should also let them know if your symptoms get worse or don't improve.
Combination therapy
In the UK, combination therapy – using levothyroxine and triiodothyronine (T3) together – isn't routinely used because there's insufficient evidence to show it’s better than using levothyroxine alone (mono therapy).
In most cases, suppressing thyroid-stimulating hormone (TSH) using high dose thyroid replacement therapy should be avoided because it carries a risk of causing adverse side effects, such as atrial fibrillation (an irregular and abnormally fast heart rate), strokes and osteooporosis and fracture.