Myxedema Secondary

Figure 3.37 Low QRS complex and T-wave voltage in all the ECG leads. This pattern, especially if it is accompanied by bradycardia, must lead one to suspect myxedema. Generalised low-voltage patterns can be seen in many other processes, in which there is a border factor that decreases the voltage secondary to cardiac causes (e.g. myocardial fibrosis in myxedema, as in this case, or pericarditis) or extracardiac causes (emphysema, pleural effusion, ascites, etc.).

The TSH stimulation is primarily a test of thyroid reserve and has three main applications (1) to identify those patients with minor impairment in thyroid function where the conventional thyroid function tests produce normal results (2) to determine whether or not a patient, who is receiving thyroid therapy for presumed idiopathic myxedema, requires this therapy (3) to distinguish between idiopathic (primary) myxedema from hypothyroidism secondary to pituitary disease. 

If a patient suspected of developing clinical hypothyroidism is investigated only with routine tests, namely total T4, FTI, and the ETR or a similar estimation, there is no way of determining whether the patient has idiopathic myxedema or hypothyroidism secondary to hypothalamic or anterior pituitary disease. The distinction should be made, however, because it is dangerous to treat pituitary hypothyroidism with thyroid hormone without simultaneously treating with adrenocortical hormones. 

Vallotton, M. B., Pretell, J. Y., and Forbes, A. P., Distinction between idiopathic primary myxedema and secondary pituitary hypothyroidism by the presence of circulating thyroid antibodies. J. Clin. Endocrinol. Metab. 27, 1-10 (1967). 

The proposition that the mechanism of action of thyroid hormones is the same as that of DNP is untenable on physiological grounds, for while DNP can raise the BMR of a myxedematous patient to normal, the other symptoms of myxedema are unrelieved (cf. Pitt-Rivers and Tata, 1959). An uncoupling of oxidative phosphorylation secondary to some other and more general action of thyroxine is, however, possible such a decreased phos-phorylating ability has, in fact, been demonstrated with mitochondria from thyrotoxic rats (Niemeyer et al., 1951 Lardy and Feldott, 1951 ... 

Hypothyroidism occurs in children (cretinism) and in adults (myxedema see Fig. 8-10). Myxedema may be primary and result from direct injury to the thyroid gland, or be secondary and due to interference with thyrotropin secretion. In contrast to primary myxedema, which is resistant to TSH injection, secondary myxedema can be corrected by thyrotropin administration. 

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