Figure 1.

(A) Schematic overview of the crosstalk between sympathetic and thyroid hormone signaling in a brown adipocyte. AC, adenylate cyclase; b3AR, beta 3 adrenergic receptor; DIO2, type II deiodinase; HSL, hormone sensitive lipase; NE, norepinephrine; PKA, protein kinase A; THT, thyroid hormone transporter; T3, 3,3′,5-triiodothyronine; T4, thyroxine; TRE, thyroid hormone response element. (B) Overview of thyroid hormone treatments used in different studies assessing its effects on brown adipose tissue. If not otherwise noted, the daily dose for duration of experiment is given. Uppercase numbers indicate the respective reference. (C) Proposed dependence of brown fat thermogenesis on thyroid hormone levels. During mild hypo- or hyperthyroidism, brown fat thermogenesis is adjusted to compensate for the change in obligatory thermogenesis and/or heat loss. In strong hypothyroidism, brown fat function fails entirely. In strong hyperthyroidism, the central T3 action dominates over the physiological adaptation and causes brown fact activation.