Starting thyroxine replacement therapy for hypothyroidism in adults

The aim of thyroxine replacement therapy for hypothyroidism is to achieve euthyroidism and relieve hypothyroid symptoms. The principles of starting replacement therapy for both primary and secondary hypothyroidism are the same, but the dose adjustment strategy is different (see Dose adjustment).

For information about when to start treatment for primary hypothyroidism in adults, see here. For information about hypothyroidism secondary to hypopituitarism, see Thyroid hormone replacement in hypopituitarism. For hypothyroidism secondary to hypopituitarism, do not start thyroid replacement therapy before excluding or correcting glucocorticoid deficiency, as this can precipitate adrenal crisis.

For thyroxine replacement therapy in children, see Hypothyroidism in children. Thyroxine therapy for thyroid cancer involves different doses and treatment targets; see Thyroxine therapy for thyroid cancer.

Thyroxine replacement therapy in adults can be started with one of two strategies:

• Initial full replacement—start therapy with a body weight–based dose that is expected to fully replace thyroid hormone requirements immediately.
• Initial partial replacement—start therapy with a low dose, and increase the dose slowly to achieve full thyroxine replacement.

Initial full replacement is preferred in most patients with overt hypothyroidism; it relieves symptoms and achieves euthyroidism more quickly, and is usually well tolerated. Initial full replacement is also used following total thyroidectomy in patients who were euthyroid before surgery.

For initial full replacement, start treatment with:

For patients at extremes of body weight, use lean body weight rather than actual body weight to calculate the levothyroxine dose¹.

Initial partial replacement is recommended for patients with only mildly elevated serum thyroid stimulating hormone (TSH) concentration or subclinical disease. It is also recommended for patients with cardiovascular disease because a high starting dose can precipitate or exacerbate cardiac ischaemia. Partial replacement may also be preferred for older patients, who typically need a lower maintenance dose than younger patients, and who are at higher risk of cardiovascular disease.

For initial partial replacement, start treatment with:

Start at the lower end of the dose range for a patient with cardiovascular disease.