Previous studies have shown that thyroid status affects the response of brown adipose tissue (BAT) to the sympathetic nervous system. For example, hypothyroidism is associated with the development of a marked synergism between α₁- and β-adrenergic pathways to stimulate type II iodothyronine 5′-deiodinase activity. Hypothyroidism also attenuates the respiratory response (thermogenesis) of isolated brown adipocytes to norepinephrine. To explore the interactions of the sympathetic nervous system and thyroid status in these cells, we compared the thermogenic and 5′-deiodinase responses to adrenergic agonists in isolated brown adipocytes from hypothyroid rats during treatment with 3,5,3′-triiodothyronine (T₃). The fivefold synergism of α₁- and β-adrenergic catecholamines to increase the deiodinase activity was progressively reduced, reaching a control euthyroid value of unity after 5 days of T₃ treatment. Hypothyroidism reduced both the O₂max (twofold to threefold) and increased the concentration of agonist required for 50% stimulation (10-fold) for both norepinephrine and forskolin. In hypothyroid cells, there was a twofold synergism between the α₁-agonist cirazoline and forskolin to increase respiration, which was blocked by prazosin and reproduced by the calcium ionophore, A23187. This synergistic effect of the α₁-agonist was lost within 2 days of T₃ administration. These studies identify a second Ca²⁺-dependent intraadrenergic synergism, which functions to ameliorate the reduced cyclic adenosine monophosphate (cAMP) responsiveness of the hypothyroid brown adipocyte.