Three selenoprotein deiodinases are involved in converting thyroxin (T4) into triiodothyronine (T3), thus the production of the active thyroid hormone is dependent on selenium status.
A case-control study by Glattre et al was published in 1989Sera from 43 persons who developed thyroid cancer on an average 4.8 years after blood sampling were compared with sera from controls. (19) Three controls per case matched for sex, age, place of residence and year of blood sampling, with regard to serum selenium and serum copper. Cases were significantly lower in serum selenium than controls, and the estimated odds ratio of thyroid cancer increased from 1 for levels greater than or equal to 1.65 mumol/l, to 6.1 for levels 1.26-1.64 mumol/l, to 7.7 for levels less than or equal to 1.25 mumol/l. When time from blood sampling to diagnosis of the case was considered, it could be shown that the protective effect of high serum selenium concentrations was restricted to the last (less than 7) years prior to the diagnosis of thyroid cancer.
Duntas remarks in a review of the role of selenium in preventing thyroid cancer that “Low Se serum levels have also been associated with increased risk of thyroid cancer and may play a role in carcinogenesis.
In 2004, the SU.VI.MAX Intervention Trial found that selenium and two other antioxidant nutrients lowered the rate of thyroid cancer in men. The incidence was only 18 per 100,000 person years in the supplemented group versus 38 per 100,000 person years in the control group. (54)
 Duntas, L.H. The role of selenium in thyroid autoimmunity and cancer. Thyroid 2006: 16;455-60.