A 42-year-old woman presents with fatigue, weight gain, cold intolerance and constipation. Examination reveals dry skin and delayed ankle reflexes. TSH is 18 mIU/L (elevated) and free T₄ is low, confirming primary hypothyroidism. She is otherwise healthy, not pregnant and has no cardiac disease. According to Australian endocrine guidance, what is the most appropriate initial levothyroxine regimen?