SUMMARY: Levothyroxine is the backbone of hypothyroidism treatment. The dosage of levothyroxine varies; however, as an estimate, an average adult patient will require 1.6 micrograms per kilogram of body weight. We present the case of a patient with hypothyroidism, controlled on a stable dosage of levothyroxine, who subsequently began semaglutide therapy for obesity. She developed rapid weight loss and presented with palpitations as her main symptoms. Both clinical and biochemical analyses demonstrated new hyperthyroidism. With the weight loss, it was deemed that her levothyroxine dosage was no longer appropriate for her new weight and was over-suppressing her thyroid function (iatrogenic hyperthyroidism), requiring a dosage reduction. With follow-up, both clinical assessment and biochemical studies noted a reduction in the suppression of the thyroid axis. This case highlights the importance of considering a dosage reduction of levothyroxine when patients lose significant weight (such as with concurrent obesity medications), to prevent iatrogenic hyperthyroidism.
LEARNING POINTS: Weight loss (pharmacological or surgical) can be associated with a reduction in TSH; it is unclear whether this is directly related to the reduction in body mass index. Hypothyroid patients on levothyroxine who are treated for obesity should be monitored for clinical and biochemical evidence of hyperthyroidism, and clinicians should anticipate that a dosage reduction may be required. The mechanism leading to iatrogenic hyperthyroidism in hypothyroid patients with weight loss therapy is unknown but believed to occur either from increased absorption of the medication or as a result of the weight loss itself (posing a supratherapeutic level of levothyroxine).
