**3. Rare concomitant disorders**

#### **3.1 Thyrotoxic periodic paralysis**

Thyrotoxic periodic paralysis (TPP) is a rare disease of the muscles secondary to hyperthyroidism presenting sudden attacks of short-term muscle weakness, stiffness, or paralysis. The underlying mechanism is malfunctions in the ion channels in skeletal muscle cell membranes: An increased influx of potassium into skeletal muscle cells leads to profound hypokalemia and paralysis. Hypokalemia in thyrotoxic hypokalemic periodic paralysis (THPP) results from an intracellular shift of potassium and not total body depletion. The symptoms may be mild or severe, and they may last for minutes or days, involving the whole body or just one or both limbs. The severity of the disease does not correlate with the hormone levels,

and muscle paralysis simply resolves by achieving the euthyroid state. TPP most frequently seen in Asian men and also reported in Hispanic adolescent males [12, 13]. Fatal outcome of a 10-years-old girl with delayed diagnosis of hyperthyroidism should draw attention to the awareness about this rare but potentially lethal disorder [14].

#### **3.2 Encephalopathy**

Presenting feature of encephalopathy in GD and Hashimoto's thyroiditis would be similar (seizures, stroke-like episodes, cognitive decline, neuropsychiatric symptoms etc.) but characteristics of thyroid derangement is reversed. Thyroid function is not an issue in Hashimoto's encephalopathy which is renamed now as 'steroid responsive encephalopathy associated with autoimmune thyroiditis (SREAT)', while brain dysfunction is associated to hyperthyroid state in GD. Hecht T and coworkers reported on a 7-year-old girl with generalized seizures, somnolence, fever, and respiratory distress. The increase of sinus tachycardia with good hydration, sufficient analgesia, and hyperthermia led to the determination of thyroid hormones, and therefore finally to the diagnosis of a thyrotoxic crisis in Graves' Disease. Symptoms were disappeared by thyrostatic therapy [15]. They concluded that thyrotoxic crisis should be considered a differential diagnosis in case of resistant unexplained sinus tachycardia, seizures, and encephalopathy.
