Hypokalaemic periodic paralysis secondary to subclinical hyp
Subclinical hyperthyroidism is defined biochemically as normal level of serum free thyroxine (T4) and triiodothyronine (T3) in the presence of a low thyroid-stimulating hormone level. It usually has no symptoms of hyperthyroidism or only has mild and non-specific symptoms. Authors report a case of subclinical hyperthyroidism presented with hypokalaemic periodic paralysis (HPP), whereby to knowledge there have been only very few cases reported in the literature. This uncommon condition is the centre of discussion in this case report.

A a patient who presented with progressive lower limb muscle weakness secondary to hypokalaemia that was refractory to potassium replacement therapy. He has no diarrhoea, no reduced appetite and was not taking any medication that can cause potassium wasting. Although he was clinically euthyroid, his thyroid function test revealed subclinical hyperthyroidism.

His 24-hour urine potassium level was normal, which makes a rapid transcellular shift of potassium secondary to subclinical hyperthyroidism as the possible cause. He was successfully treated with potassium supplements, non-selective beta-blockers and anti-thyroid medication.

Learning points
- Although hypokalaemic periodic paralysis is more common in hyperthyroidism, this case report showed that it can also occur in subclinical hyperthyroidism.

- A very high index of suspicion is needed among any treating physicians to diagnose this condition. This is because even in a patient with hyperthyroidism, the thyrotoxic symptoms can be absent during hypokalaemic periodic paralysis attack in 10%–25% of the patients. This makes the diagnosis very challenging, more than it already has.

- Treatment of hypokalaemic periodic paralysis secondary to hyperthyroidism involves prompt potassium replacement via oral or intravenous route depending on the level of potassium deficit and also the severity of weakness.

- It is important to remember that rebound hyperkalaemia can occur during the replacement period, and fatal cardiac arrhythmias have been reported as a result of it. Therefore, serum potassium has to be closely monitored during the treatment.

- Apart from potassium replacement, it is necessary to start on non-selective beta-blockers and anti-thyroid medication in this condition in order to prevent further episode of recurrent hypokalaemic periodic paralysis, even in subclinical hyperthyroidism.

Source: https://casereports.bmj.com/content/14/6/e240666?rss=1