L-carnitine and thyroid hormone tended to antagonize reciprocally in human body. Urinary excretion of L-carnitine decreased in hypothyroid patients, and levothyroxine supplementation increased excretion of L-carnitine. The investigators hypothesized that supplying L-carnitine to hypothyroid patients with fatigue symptom could improve the quality of life, and fatigue score in them. Therefore, the investigators planned to compare the efficacy of L-carnitine and placebo in hypothyroid patients who had taken levothyroxine.

Inclusion Criteria:

• Clinical diagnosis of hypothyroidism with fatigue symptom
• Currently use of the same amount of levothyroxine over at least six months
• Normal serum free T4 level

Exclusion Criteria:

• Current smoker
• Serum hemoglobin level of under 12mg/dL
• Clinical diagnosis of diabetes mellitus or fasting serum glucose level ≥ 126mg/dL or HbA1c level ≥ 6.5%
• History of significant heart failure
• History of large amount of alcohol consumption
• Uncontrolled hypertension (systolic blood pressure ≥ 160 millimeter of mercury (mmHg) or diastolic blood pressure ≥ 100 mmHg
• pregnant, or planning to be pregnant, or breast feeding women
• aspartate aminotransferase (AST) or alanine aminotransferase (ALT) ≥ 2 times of normal limit
• serum Cr level > 2.0mg/dL