Original Article: Hypothyroidism: Diagnosis and Treatment

Issue Date: May 15, 2021

See additional reader comments at: https://www.aafp.org/afp/2021/0515/p605.html

To the Editor: Dr. Wilson and colleagues' excellent article provided a recommendation for the starting dosage of levothyroxine to treat overt hypothyroidism. In nonpregnant patients younger than 60 years with no cardiac disease, mental changes, hypothermia, or stupor, the article recommended initiating a levothyroxine dosage of 1.5 to 1.8 mcg per kg per day. The references cited for that dosage recommended different amounts, including 1.6 mcg per kg per day, 1.6 to 1.8 mcg per kg per day, and 1.6 mcg per kg per day for an average-sized man or woman.^1,2

Current guidelines recommend a levothyroxine dosage of 1.6 mcg per kg per day based on ideal body weight or lean body mass instead of actual body weight.^3,4 Ideal body weight can be calculated as the weight for height that would generate a body mass index of 24 to 25 kg per m². Lean body mass is a better predictor of the dosage requirement than actual body weight.⁵ Without this clarification, some patients may receive a dosage of levothyroxine that is too high.

In Reply: We thank Dr. Muncie for the opportunity to clarify the starting dosage of levothyroxine in patients with overt hypothyroidism. The need for a dosing adjustment based on weight is clinically relevant because of the high prevalence of obesity among adults in the United States.

Although a starting dosage of 1.6 to 1.8 mcg per kg per day can be appropriate in patients with a body mass index of less than 26 kg per m², weight-based dosing of levothyroxine may inappropriately overdose patients who are overweight and obese.^1,2 A lower starting dosage may help bring patients' thyroid-stimulating hormone (TSH) levels into normal range more quickly. Currently, there is no clear, high-level guideline on how to best adjust the starting dosage in patients with a body mass index greater than 30 kg per m². Strategies include using the ideal body weight or considering the body mass index as a coefficient. Monitoring TSH levels every six to eight weeks and adjusting levothyroxine dosages until the TSH reaches goal are critical to avoid iatrogenic hyperthyroidism or under-replacement. As noted in the article, it is important to regularly monitor the TSH level until it is in the normal range, then make future adjustments based on symptoms or periodic testing.