The Challenges and Complexities of Thyroid Hormone Replacement

Shayri M. Kansagra, BS; Christopher R. McCudden, PhD; Monte S. Willis, MD, PhD

Disclosures

Lab Med. 2010;41(6):229-348. 

In This Article

Pharmacology of Thyroid Hormone Therapy

In standard replacement therapy, T4 is given orally at doses of 1.6 μg/kg/day; this translates to a dose of 120 μg/day in a 75 kg adult. However, doses range from 50–200 μg/day in efforts to balance the risk for hyperthyroidism with clinical symptoms of hypothyroidism.[30] Dosage also depends on the cause of hypothyroidism, where individuals with total thyroidectomy will need higher doses of T4 than those with mild Hashimotos' thyroiditis. Once ingested, roughly 80% of a given dose of T4 is absorbed into the body;[31] this too is variable depending on the timing of food intake.[32] Drug formulation is also a consideration, where generic T4 may have slightly different additives than brand name preparations affecting absorption.[33] Although studies have shown equivalence,[34] it is recommended that patients stay with the same brand over the course of therapy.[35] As discussed above, there are also pure T3 formulations (Liothyronine), combination T4/T3 preparations (Liotrix; mixture of T4/T3 at a 4:1 ratio), and animal extracts containing T4 and T3 (Thyroid United States Pharmacopeia). These preparations are also subject to brand and even lot to lot variability, particularly in the case of porcine thyroid extracts.[36]

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