T4 is a storage form of thyroid hormones. T4 cannot be used anywhere in the body at all, so it is considered inactive or storage. It can only be converted to an active hormone state (T3) before being used.
Some T3 is made by the thyroid; the rest is converted from T4 in places like the liver and kidneys before being circulated for use around the body, also in cells that convert T4 to T3 for their own use directly.
T3 is also in a prescription called cytomel or liothyronine, this is synthetic T3. Cytomel is often sold in 5 mcg (low amount) or 25 mcg (high amount) pill forms so it is difficult to titrate a long term dosage sometimes if you need to be on it. T3 is also in natural thyroid (NDT) prescriptions (from animals). There are 9 mcg of T3 in each grain of natural thyroid in USA. T3 has very rapid effects. It has a fairly short half life.
T4 is in a prescription called Levothyroxine, Synthroid, Tirosint and other names. These are the synthetic forms. It also is in natural thyroid prescriptions (from animals). In natural thyroid there are 38 mcg of T4 per grain of natural thyroid (NDT) in USA. T4 tends to change things slowly because it is converted to T3 before being used, and may take weeks to come out of the body and thus has a longer half life.
T3 is best measured by Free t3 which is a measure of how much T3 is available for use. This varies fairly widely over the day. A higher value means more is available. A low value and we generally do poorly at that point of the day where it is low. There are other T3 measures other than the Free T3 form but they are not ideal. T3 labs don’t measure daytime levels well near time of taking medications based on experience, so Free t3 labs are fairly time sensitive. When Free t3 level is low, many of us experience severe hypo symptoms no matter what the TSH says. You can also experience too much T3 and get hyper symptoms so having an experienced physician handling prescriptions in the T3 or NDT area is ideal, unfortunately this is hard to find.
T4 is best measured using Free t4. When relatively high in some of us it excites a high level of hyper symptoms and I experience this fairly easily. I do poorly at high levels of Free t4. One can also be hypo or get hypo symptoms when the FT4 is quite low.
rT3: Reverse t3 is a primal response of our bodies sensing something wrong, and forcing a slowing of metabolic rates in an attempt to create a somewhat hidden self preservation mode of sorts. Basically your body attempts to slow metabolism when it senses you are ill, deficient in iron (ferritin), minerals, vitamins, other hormones, or otherwise have something off or wrong. Having a high metabolism when ill would not be a good thing in general, so we developed an internal system to slow metabolism at these times. Symptoms sometimes may include hair loss and fatigue not resolved by raising FT3 alone. Basically rT3 blocks T3 receptors all over the body and the Free t3 can't get in. So you will feel quite poor, sometimes even presenting with a good or very good FT3. rT3 looks similar to T3 to the body but it can't function like T3 at all. It blocks the active thyroid (T3) hormone from being used since receptors are blocked up by the rT3 molecule. We then expend less energy, but we won’t feel well. All people have rT3, one can never get rid of all of it. However when rT3 is really high, that is when our body has kicked into its mode of forcing a slowing metabolism. We can potentially correct situations of high rT3 by finding the source of the problem.
Two common problem areas and some symptoms often faced post TT
As thyroid cancer survivors, having no thyroid, we are fairly unique medically in that we can experience both an excess (hyper) and a deficiency (hypo) simultaneously.
HYPER (most common with a high FT4 post TT on high levels of T4, but can be from too high a FT3 too) symptoms include mood changes and swings, A mean irritability or a short temper, headaches and migraines, anxiety, palpitations, shortness of breath and/or breathlessness, too fast a GI system (initially) or changes in GI system with eventual constipation from dehydration in some cases, sleeplessness and restlessness, more of a sharp body ache or pain like joint pain or sharp shoulder pain issues, weight changes due to excess hunger with little or no satiety, lots of pre-migraine and migraine aura effects (can include thyroid ocular aura disorders -TOADs) even if you never get or have never had a migraine in your life.
HYPO (most common with low FT3 even if TSH suppressed) all the symptoms of hypothyroidism, but common are the brain fog, depression, loss of attention span (forgetting things), skin issues, hair loss, fingernail issues, feeling dog tired, Sleeping lots, slowing metabolic rate leading to inability to lose weight despite honest effort, Tired irritability, more of a dull and swollen body ache or pain like swollen joint pain, irritable/sensitive GI, inability to do things like we used to, constipation or too slow a GI system, blooming allergies, just can't focus on work or family. When very HYPO a differing kind of irritability or a "tired irritability" and a differing headache than the HYPER one. A very low Free T3 can also cause bradycardia, palpitations in some, and many types of cardiac issues are possible. Many more issues are common when HYPO and can be found on internet searches.