Hypothyroidism is associated with  increased systemic vascular resistance, an increased arterial stiffness, decreased cardiac contractility, decreased cardiac output, atherosclerosis, coronary artery disease, bradycardia and conduction abnormalities; impairment left ventricular diastolic function characterized by a slowing of myocardial relaxation and impaired early ventricular filling and pericardial effusion (PE) .

Hemodynamic, mechanical and metabolic causes of decreased ECG voltage:

• Low ECG voltage may be due to combination of both severe thyroid hormone deficiency and large PE ( the “damping” effect of increased layers of fluid between the heart and the recording electrode)
• Decreased resting HR, cardiac output, heart contractility and stroke volume  identified in hypothyroidism. As the contraction of heart muscle is associated with QRS complex,  QRS  voltage and HR are reduced in hypothyroid patients.
• Hypothyroidism leads to a decreased synthesis of albumin. Increased permeability of the capillaries results in the loss of albumin in the intravascular compartment through increased transcapillary escape rate of albumin, eventually leading to reduced plasma volume. Thus, the consequent increase in the concentration of the albumin in extravascular compartment causes increased interstitial fluid volume and impaired lymphatic drainage (Interstitial edema with alterations in the structure of the interstitium, reduced interstitial compliance, and disturbance in transcapillary fluid balance vs  contrary the intracellular lysosomal storage when ECG voltage is increased) 
• Possibly and further lower colloid osmotic pressure

I agree with Tatiyana'S answer. PE, decreased thyroid hormone, decreased cardiac contractility, low serum albumin and osmotic pressure are important contributors.Associated obesity which is very common with hypothyroidism may be an important cause of low voltage complex. Interestingly low voltage complex is commonly seen in patients more than 60ys age and commonly associated with sinus bradycardia.. Thanks... 

https://www.ncbi.nlm.nih.gov/pubmed/4057554

Thank you all for such full answers!

 I agree with Tatiyana Vaikhanskaya

Thank you @Tatiyana. SPOT on response.

In Uganda, A PUBLICATION is out associating LOW QRS voltage as a predictor of Mortality & Poor Prognosis in the Chri=nically & Very Sic. We Attribute LOW QRS Voltage to THYROID DERRANGEMENTS that happen in Chronicall oo CRiticall SICK people. Again Thanks #Tatiyana  for the EXCELLENT OVERVIEW

In addition to increased insulation by effusion or myxedema, generated electrical energy is also reduced.  Thyroid hormon is the energetic gear of whole organs and the heart as well. The decreased contractile performance induced by hyperthyroidism is largely mediated by decreased expression of the calcium adenosine triphosphatase (ATPase) of the sarcoplasmic reticulum  thus the electrical activity. İon channels are also affected and so the meberane potetials

gREAT RESPONSE @SERKAN DUYULER

I agree with Tatiyana

1) Pericardial effusion (Increasing the impedance to current flow).

2) IHD with interstitial fibrosis & loss of myocytes caused by CAD resulting from hypercholesterolemia (++ LDL-C)

3) Myxedema with deposition of the mucopolysaccharides in subcutaneous tissues impedes current flow to the surface ECG