What is the recommended dosage of thyroxine (T4) for adults with hypothyroidism?

Levothyroxine Dosing for Adult Hypothyroidism

For adults with hypothyroidism, start levothyroxine at 1.6-1.8 mcg/kg/day (typically 100-125 mcg/day for a 70 kg adult) if under 50 years old and otherwise healthy, or 25-50 mcg/day if over 50 years or with cardiac disease, titrating every 6-8 weeks to normalize TSH. 1, 2

Initial Dosing Strategy

For Younger, Healthy Adults (<50 years)

• Begin with full replacement dosing at approximately 1.6-1.8 mcg/kg/day, which translates to 100-125 mcg/day for a typical 70 kg adult 1, 3, 4
• This full-dose approach is appropriate for otherwise healthy individuals under 50 years who have no cardiovascular disease 1
• Doses greater than 200 mcg/day are seldom required, and inadequate response to ≥300 mcg/day suggests poor compliance, malabsorption, or drug interactions 1

For Older Adults (>50 years) or Those with Cardiac Disease

• Start with 25-50 mcg/day and increase gradually at 6-8 week intervals 1, 2
• For elderly patients with cardiac disease, an even lower starting dose of 12.5-25 mcg/day is recommended, with gradual increments at 4-6 week intervals 1
• Older patients typically require less than 1 mcg/kg/day for maintenance 1, 5
• This cautious approach prevents exacerbation of underlying cardiac conditions, particularly coronary artery disease and arrhythmias 2, 6

Dose Titration Protocol

Adjustment Increments

• Adjust levothyroxine in 12.5-25 mcg increments based on TSH response and clinical status 1, 2
• For younger patients without cardiac disease, 25 mcg increments are appropriate 2
• For elderly or cardiac patients, use smaller 12.5 mcg increments to minimize cardiovascular risk 2

Monitoring Schedule

• Recheck TSH and free T4 every 6-8 weeks during dose titration until target TSH is achieved 2, 1
• Once stabilized on an appropriate dose, monitor TSH every 6-12 months or when symptoms change 2
• Allow 4-6 weeks between dose adjustments to reach steady state, as levothyroxine has a long half-life 1

Target TSH Levels

• Aim for TSH within the reference range of 0.4-4.5 mIU/L for most patients with primary hypothyroidism 3, 2
• For secondary (pituitary) or tertiary (hypothalamic) hypothyroidism, titrate to restore free T4 to the upper half of the normal range rather than using TSH 1, 7

Special Populations and Considerations

Severe Hypothyroidism

• Start with 12.5-25 mcg/day and increase by 25 mcg every 2-4 weeks with close clinical and laboratory monitoring 1
• This gradual approach prevents cardiovascular complications in severely hypothyroid patients 1

Patients with Adrenal Insufficiency

• Always start corticosteroids several days before initiating thyroid hormone replacement to prevent precipitating adrenal crisis 7, 2
• This is critical in patients with hypopituitarism or immune checkpoint inhibitor-induced hypophysitis 7

Pregnancy

• Women with hypothyroidism who become pregnant should increase their weekly levothyroxine dosage by 30% (take one extra dose twice per week) immediately upon pregnancy confirmation 4
• Monitor thyroid function monthly during pregnancy and adjust accordingly 4

Administration Guidelines

• Take levothyroxine in the morning on an empty stomach, at least 30-60 minutes before food 1
• Separate levothyroxine by at least 4 hours from medications that interfere with absorption (calcium, iron, proton pump inhibitors) 1, 2

Common Pitfalls to Avoid

Overtreatment Risks

• Approximately 25% of patients on levothyroxine are inadvertently maintained on doses that fully suppress TSH 2
• Overtreatment increases risk for atrial fibrillation, osteoporosis, fractures, and cardiac complications 2, 8
• A suppressed TSH (<0.1 mIU/L) with elevated free T4 indicates tissue thyrotoxicosis and requires dose reduction 8, 2

Undertreatment Consequences

• Persistent hypothyroid symptoms, adverse cardiovascular effects, abnormal lipid metabolism, and reduced quality of life result from inadequate dosing 2
• TSH persistently >10 mIU/L despite treatment indicates insufficient replacement and requires dose escalation 2

Age-Related Dosing Errors

• Older patients (>60 years) typically require lower maintenance doses than younger adults, often 100 mcg/day or less 5
• Some elderly patients may need only 50 mcg/day for adequate replacement 5
• Failure to reduce doses in elderly patients increases risk of iatrogenic hyperthyroidism 5

Reassessment Triggers

• Reduce or discontinue levothyroxine if TSH becomes suppressed, as this may indicate overtreatment or recovery of thyroid function 2
• For patients with persistent symptoms despite normalized TSH, reassess for medication adherence, malabsorption, drug interactions, or other causes of symptoms 1, 4
• Consider endocrinology referral for patients requiring doses >200 mcg/day or with inadequate response to appropriate dosing 1