Hypothyroidism is the most frequent endocrine condition if you exclude diabetes mellitus. In the broad sense it is due to the gradual reduction or complete cessation of thyroid hormone output from your thyroid gland.
Hormones from your thyroid.
Your thyroid gland puts out two main hormones T4 (levothyroxine) and T3 (liothyronine). If you have surgery or ablation due to radioiodine. it is likely that both these hormones will stop abruptly. This is quite easily recognized and hormone replacement will be started. However in immune attack (hashimotos) or viral attack (thyroiditis), the impact on the thyroid is not quickly picked up