Brown fat produced T3 and the enzyme that converts T4 to T3
"Brown adipocytes are known to produce biologically active molecules that modulate thermogenesis, such as thyroid hormone (triiodothyronine, T3), retinaldehyde, and retinoic acid. Thyroid hormone has been recognized as an important regulator of thermogenesis. The expression of type II thyroxine 5′-deiodinase (Dio2), which converts thyroxine to T3, is restricted to BAT and strongly induced during BAT activation [87]."
https://www.cell.com/trends/endocrinology-metabolism/fulltext/S1043-2760(15)00046-6
Most of the T3 in BAT is produced locally (55%), hence BAT seems to be a significant source of T3.
Comparable to the liver in terms of numbers of the receptor.
"Fifty-five percent of the nuclear T3 was generated locally, and 45% was derived from circulating T3. BAT is, hence, comparable to the liver in number of receptors (-5000/ cell) and to the pituitary with regard to saturation and relative contributions of locally generated T3 and plasma T3 to nuclear T3. These results suggest that BAT may be an important target for thyroid hormones and, along with other data, that alterations in the activity of the type II 5′-deiodinase of this tissue may influence the saturation of nuclear T3 receptors. (Endocrinology120: 55–62,1987)"
https://academic.oup.com/endo/article-abstract/120/1/55/2540567
"These results indicate that BAT can be a major source of plasma T3 under suitable circumstances such as acute or chronic exposure to cold."
https://www.jci.org/articles/view/112239
--> The fact that BAT can strongly increase overall plasma T3 levels for significant spans of time is remarkable.
And, it can also be increased/activated by certain supplements. It doesn't necessarily have to be cold exposure.
If one finds a benign supplement that does that, I could see that replacing a thyroid supplement for people that can't access it or can't make it work.
BAT activation also increases liver health and insulin sensitivity, which complements increased T3 and should make people more thyroid sensitive/less thyroid resistant.
