Thursday, March 16, 2017

Pathophysiology of myopathy caused during hypothyroidism and hyperthyroidism

Hello awesomites! Today's topic of discussion is - Myopathy in thyroid disease.
Interestingly, it is caused by both, hypothyroidism as well as hyperthyroidism.

What is myopathy?
It is a disease of muscle tissue where, ultimately, muscles get weak and are unable to perform work due to deficiency of ATP.

Why is there myopathy in hyperthyroidism?
The thyroid hormone is a catabolic hormone. Hyperthyroidism increases energy expenditure, glucose turnover, lipolysis, and protein breakdown (proteolysis). But here is the catch - Hyperthyroidism increases whole-body protein turnover and breakdown before any measurable changes in energy expenditure or glucose and fat metabolism, suggesting that amino acid and protein metabolism is an early and primary target for thyroid hormone action in humans. It was therefore concluded that the thyroid-hormone concentration may be an important factor in regulating muscle proteolysis. The altered protein metabolism causes myopathy.

Then, one may ask, why myopathy in hypothyroidism? Less thyroid hormone should lead to less protein breakdown, shouldn't it?

Well, this is a good question! Slightly complex and tricky to answer though. 

In hypothyroidism, there is abnormal glycogenolysis, defective mitochondrial oxidative metabolism and triglyceride storage.

Abnormal glycogenolysis and triglyceride storage: Less glucose is released and utilised because of this. The body starts using more proteins usually derived from muscles leading to myopathy.

Mitochondrial oxidative metabolism defect: Thyroid hormone is responsible for activation of bc1 complex also known as complex 3 & succinate dehydrogenase. Less activation of bc1 leads to less formation of ATP from glucose.... So again, the body switches to proteins from muscles as a source of energy!

That's all!
Stay cool :)
~Ojas

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