Size of quadriceps femoris may contribute to thyrotoxic periodic paralysis

Thyrotoxic periodic paralysis (TPP) frequently occurs on male individuals at their third and forth decades. The major site of involvement is the proximal muscles of lower limbs. Increasing evidence has shown that the occurrence of TPP is determined by multiple factors. We hypothesized that apart from hormonal fluctuations, skeletal muscle itself may explain for the age and sex variance as well. Our study was established to explore whether the size of lower limb skeletal muscles were related to TPP. We conducted a clinical experiment including 43 patients diagnosed with TPP (Group 1) and 39 pure hyperthyroidism individuals (Group 2). Current age, body mass index (BMI), thyroid stimulating hormone (TSH), free triiodothyronine (FT3), free thyroxine (FT4), average girth of bilateral upper arm and thigh, physical activity level (PAL) were measured. We also adopted B mode ultrasound to quantify the muscle thickness (MT) of the major muscle involved in the disease, the quadriceps femoris (QF, including rectus femoris, RF; vastus intermedius, VI; vastus medialis, VM and vastus lateralis, VL). Patients were matched in TSH, FT4 and FT3. PAL was also statistically identical between groups. Age, BMI, thigh girth, the average of bilateral MT of QF were statistically different. After adjusting for age, BMI and girth, Group 1 still presented with larger MT of QF than Group 2, regardless of their current thyroid hormone level. There indeed exists an independent relationship between muscle thickness and TPP.