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Hypo is related to atherosclerotic coronary artery disease in 2 ways. First,
its metabolic and hemodynamic characteristics, i.e., hypercholesterolemia
and hypertension, increase the risk of atherogenesis (82,83). Second, hypo
creates a relatively negative chronotropic and inotropic state in which
there is diminished myocardial oxygen demand, and recovery from which may
provoke underlying coronary ischemia. The most clinically important consequence
of T4 deficiency on lipoprotein metabolism is elevation of circulating
LDL-Cholesterol concentrations. Overt and subclinical hypo are commonly
diagnosed in patients referred for management of hypercholesterolemia.
Hypertriglyceridemia and impairment of fatty acid mobilisation are also
associated with hypo. The occurrence of increased risk factors contrasts
with the relatively low incidence of myocardial infarction or angina pectoris
in the hypo patient population. This discrepancy is most likely explained
by the decreased metabolic demand placed on the myocardium. Nevertheless,
the risk that T4 therapy of hypo will exacerbate myocardial
ischemia in patients with coronary artery disease is widely appreciated,
as is the universal recommendation that T4 therapy be initiated
in a low dose and escalated in small increments in the management of such
patients. However, there is little clinical research addressing this issue.
Among 55 hypo patients with coronary artery disease, T4 and/or
T3 exacerbated heart disease in only 9, while the remainder
were unchanged or improved (84). Although the increases in heart rate and
contractility that occur with T4 therapy augment myocardial
oxygen demands, simultaneous reductions in ventricular dimensions (related
to preload) and diastolic BP (afterload) may be important beneficial effects
in some patients. In a recent work, angiographic coronary artery disease
progression could be prevented by adequate T4 replacement in
hypo (85). Thus, thyroid hormones can protect against atherosclerosis,
presumably due to their metabolic effects on plaque progression.
The hypo patient with unstable myocardial ischemia presents a special challenge, particularly when coronary vascular interventions are indicated. Risks of exacerbating myocardial ischemia must be balanced against those of surgery or angioplasty in the hypo state. No increased risk of perioperative death has been observed in hypo patients undergoing coronary revascularization (86). However, higher incidences of intraoperative hypotension and perioperative heart failure were observed. For percutaneous transluminal angioplasty, success rates and risk of complications were comparable in hypo and euthyroid patients, although there was a trend toward higher risk of hematoma formation in the hypo group (87). |
