Dose adjustment in primary hypothyroidism

After starting thyroxine replacement therapy for primary hypothyroidism (with either full or partial replacement), measure serum TSH concentration and adjust the levothyroxine dose at 4- to 8-weekly intervals to achieve a serum TSH concentration in an age-specific target range. Subtle dose adjustments can be achieved by prescribing different strength tablets on different days of the week.

When the serum TSH concentration is in the target range, the serum free thyroxine (T4) concentration is usually in the upper half of the normal range. A serum TSH concentration below the target range indicates overtreatment; long-term overtreatment can cause atrial fibrillation and accelerated loss of bone mass. For patients who have an inadequate response to levothyroxine (eg persistently elevated TSH, continued symptoms despite normalised TSH), see Inadequate response to thyroxine replacement therapy.

For patients younger than 60 years, aim for a serum TSH concentration between 0.5 and 2.5 milliunits/L, which approximates the median serum TSH concentration in the normal adult population.

In older patients, the serum TSH target range is less aggressive and the final levothyroxine dose is typically lower, because:

  • thyroid hormone requirements decrease with age
  • the normal serum TSH concentration rises with age (most markedly in people aged over 80 years)
  • older patients are more susceptible to the consequences of overtreatment
  • the risk of provoking unrecognised cardiac ischaemia is higher in older patients.

For patients 60 years and older, a reasonable serum TSH target range is 1 to 5 milliunits/L.

For patients older than 80 years, consider an even higher serum TSH target range (eg 4 to 6 milliunits/L).

For frail elderly patients and patients with biochemically severe hypothyroidism at baseline, base initial dose adjustments on clinical response rather than serum TSH concentration. Do not aim for normalisation of serum TSH concentration within the first 3 months of treatment. If symptoms of cardiac ischaemia occur, do not increase the dose further until the cardiac condition has been investigated and managed.

Full replacement may not be achievable for a patient with severe cardiovascular disease who cannot undergo coronary artery revascularisation; a degree of persisting hypothyroidism is sometimes necessary in these patients.

Multiple brands of levothyroxine tablets are currently available in Australia. Minor dose adjustments may be required if switching between brands that are not bioequivalent.