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The thyroid hormone is fuel for the body, it regulated many function of the body and it is produced by thyroid gland situated in the neck. When it is not released in sufficient quantity tsh
goes high and it means you are having deficiency of this hormone in your body. so you need to take supplements and that is the tablet given to you. This is known as hypothyroidism
and per se it is a condition which can be very well be controlled with regular medication and without any major issues so do not worry.