1. Abnormalities in skeletal muscle function were characterized in 19 hypothyroid subjects and the extent and time course of recovery in these abnormalities after treatment determined.
2. Slow muscle relaxation was observed on examination in 11 patients and 12 out of 16 patients had a reduced quadriceps maximum relaxation rate (MRR) and 11 out of 15 had a prolonged ankle jerk relaxation time (AJRT).
3. Measurements of quadriceps force revealed significant muscle weakness when comparison with 15 age matched healthy subjects was made (P<0.001). This weakness was often not evident on clinical examination.
4. A raised plasma creatine kinase (CK) activity was characteristic.
5. With treatment the plasma CK activity and the AJRT rapidly became normal often long before the serum thyroid-stimulating hormone (TSH) was normal, but the quadriceps MRR was slower to recover. It was rarely normal before the serum TSH became normal. Weakness was slow to recover and seven patients remained weak at the end of the study despite being euthyroid for a mean period of 1 year, but strength increased modestly overall (P < 0.01). The urinary creatinine/height index was unchanged, indicating that total muscle mass does not alter with therapy.