Hypothyroidism is a classical example of generalised edema formation . The mechanism of which is extensively studied .Still we are not clear about it .
Content of edema fluid
- Mucopolysacharides -Hyalinorunic acid
- Albumin
Mechanism
- Albumin leak into interstitial and extra cellular space due to capillary dysfunction.
- Reduced lymph clearance – probably due to poor lymphatic tone .
Why hypothyroid edema does not pit on pressure ?
For pitting to occur tissue should be compressed and retain the elasticity .This means the fluid can escape into the cell when mechanically compressed and comes back when the elasticity of skin brings it back to its original status. For this to occur the skin and subcutaneous matrix should be normal in texture.
In cardiac failure and renal failure edema is primarily due to altered hydrostatic forces and skin is intrinsically normal .So some amount of pitting is retained . In hypothyroidsim and lymphedema where there is an intrinsic pathology of the skin pitting is rare.
Why constriction and cardiac tamponade are rare with hypothyroid pericardial effusion ?
Like the generalised slow response of hypothyroid individuals the effusion is also very slow forming and is rarely large so tamponade is rare .
The hypothyroid infiltrates which collects within the pericardial space it rarely infilitrates the fibrinous pericardium (The thick outer shell ) .Unless fibrinous pericardium is inflamed or infiltrated constriction is exceedingly rare .Simple epicardial infiltration may cause some restrictive physiology but not constriction .
Reference
Lancet. 1975 Mar 8;1(7906):564-6.Effusions into body cavities in hypothyroidism. http://www.ncbi.nlm.nih.gov/pubmed/47029
Dr.S.Venkatesan MD 
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