Thyroid hormones appear to increase catabolism of vitamin K-dependent clotting factors. Patients stabilized on oral anticoagulants may require dosage adjustment, through close monitoring of prothrombin time, when thyroid hormone therapy is started. No special precautions appear to be necessary when oral anticoagulant therapy is begun in a patient already stabilized on maintenance thyroid therapy. Thyroid hormones increase the adrenergic effect of catecholamines such as epinephrine and norepinephrine. Therefore, injection of these agents into patients receiving thyroid preparations increases the risk of precipitating coronary insufficiency, especially in patients with coronary artery disease.
Careful observation is required.
Cholestyramine binds orally administered thyroid hormones in the intestine, impairing their absorption. In vitro studies indicate that the binding is not easily overcome. Thyroid hormonal replacement increases metabolic rate, which requires an increase in digitalis dosage. Initiating thyroid replacement therapy may require increases in insulin or oral hypoglycemic requirements. The effects seen are poorly understood and depend upon a variety of factors such as dose and type of thyroid preparations and endocrine status of the patient.