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.
Two common problem areas and some symptoms often faced post TT
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, irritability or a short temper, headaches and migraines, anxiety, palpitations, shortness of breath and/or breathlessness, too fast a GI system and in a rarer few somewhat on the constipation side, sleeplessness and restlessness, more of a sharp body ache or pains like joint pain or sharp shoulder 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, 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, food intolerance, allergies, just can't focus on work or family, when very HYPO a differing kind of irritability and 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.