What element is necessary for the production of thyroid hormone?

Iodine

Explanation:

This is a trace element needed for the  synthesis of the two thyroid hormone ; T4- thyroxine and T3-triiodothyronine. Four iodine molecules are added in T4 synthesis, and  three for T3 synthesis. Generally T4 is the major hormone produced, but  biologically transformed into active form T3.

Iodine majorly and selenium are the two trace elements needed for thyroid hormones synthesis. Therefore required amount of iodine is important in daily diet. iIn adequate supply of this leads to under production of iodine called hypothyroidism. poor brain development ( especially in kids),and goiter.

Sea foods . eggs, poultry seaweeds are rich sources of iodine.

Conversely, when  excessive amount is present in the blood, autoimmune thyroid diseases can occur,including cancer developments in the thyroid gland due to hyper cell activities.