If you have your entire thyroid removed you then need to take replacement thyroxin for the rest of your life, which can have its own set of side effects.
Synthetic thyroxin is also often used for those that have a poorly functioning thyroid. This can be effective for some, but others report side effects after long term use.
It is inactive T4 and requires the thyroid to convert it to T3.
a common brand is Synthoid
Natural forms of thyroxin are also available, often desiccated (dried) and may prove more effective as they contain both T4 and T3.
common brands are:
Armour
PTU