Myocardial I-123-MIBG kinetics in acutely hypothyroid patients with differentiated thyroid carcinoma

Marked changes in cardiac function have been noted in patients with hyperth yroidism or hypothyroidism due both to changes in sympathetic system functi on and to biochemical modifications of myocardial tissue. Metaiodobenzylgua nidine (MIBG), an analogue of norepinephrine, can be used to evaluate myoca rdial sympathetic tone. Here, we report myocardial I-123-MIBG kinetics in p atients with differentiated thyroid carcinoma undergoing acute hypothyroidi sm followed by hormonal replacement as part of their routine clinical follo w-up. Ten patients with differentiated thyroid carcinoma in acute hypothyro idism (A) and on hormonal replacement with thyroxine (150 mu g.day(-1)) and triiodothyronine (20 mu g.day(-1)) (B) underwent scintigraphic imaging 20 min and 4 h after injection of 111 MBq of ultra-high specific activity I-12 3-MIBG. NO patient had cardiac disease or was taking medications that could interfere with cardiac or autonomic system function. Cardiac MIBG kinetics (heart-to-mediastinum, H/M, ratio and myocardial washout rate), serum nore pinephrine, T3, T4, FT3, FT4, TSH, CPK, CPK-MB, blood pressure and ECG were evaluated. Systolic and diastolic blood pressure did not differ significan tly between state A and state B. In the acute hypothyroid state (A), the pr evalence of non-specific ST-T abnormalities was 70% and heart rate was sign ificantly different (P < 0.001) than in state B. Norepinephrine and CPK-MB levels were higher during hypothyroidism, but this did not reach statistica l significance. A positive correlation between early H/M and delayed H/M in the hypothyroid state (r = 0.57) and an even higher positive correlation b etween early H/M and delayed H/M in the euthyroid state (r = 0.91) were see n. The myocardial and mediastinal MIBG washout rates were significantly dif ferent between the hypothyroid and euthyroid states (P < 0.05), whereas the lung washout rate did not differ significantly between the two metabolic s tates. We conclude that the myocardial washout rate during hypothyroidism i s clearly increased (P < 0.005) with a subclinical derangement of myocardia l adrenergic innervation, which is rapidly reversed with hormonal therapy. ((C) 1999 Lippincott Williams & Wilkins).