Thyroid replacement therapy

Levothyroxine is a common medication physicians prescribe for patients with primary hypothyroidism. Levothyroxine is synthetic thyroxine (T4) which is deiodinated in peripheral tissues to form T3 which is more active. 

The initial dosing of Levothyroxine for young healthy adults is approximately 1.6 mcg/kg body weight per day. However, studies suggested that lean body mass may better correlate with daily T4 requirements. The current weight-based dosing of levothyroxine may have inappropriately overdoses overweight and obese patients.

Other considerations for the initial dose are the patient's demographic, duration of hypothyroidism, and cardiac history information. For elderly patients and patients with known coronary heart disease, Levothyroxine should be started at a lower dose usually 25-50mcg PO daily. For patients with short-duration hypothyroidism (e.g., less than 2 months), the initial dose is approximately 1.2mcg/kg/day which is 75% of the anticipated dose needed. 

During follow-up visits, it's important to assess the patient's adherence and knowledge of Levothyroxine. The medication should be taken on an empty stomach 1h before food/meds. It should be several hours apart from PPI, iron, calcium carbonate, aluminum hydroxide, and cholestyramine. 

For follow-up visits, it's also important to check TSH q6 weeks and adjust by 12-25mcg until normal TSH. Once the maintenance dose was identified, the patient should be assessed once yearly with TSH or sooner if patient developed new symptoms.

Reference:

Lean body mass is a major determinant of levothyroxine dosage in the treatment of thyroid diseases - PubMed (nih.gov)

Evaluation of Thyroid Hormone Replacement Dosing in Overweight and Obese Patients After a Thyroidectomy - PubMed (nih.gov)

Treatment of primary hypothyroidism in adults

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