Levothyroxine and Metformin (combination)

Levothyroxine is a synthetic form of the thyroid hormone thyroxine (T4) primarily used to treat hypothyroidism, a condition where the thyroid gland cannot produce sufficient quantities of thyroid hormones. It's also used as an adjunct to surgery and radioiodine therapy in managing thyrotropin-dependent well-differentiated thyroid cancer. Metformin is an oral antidiabetic medication that works by decreasing glucose production in the liver and improving insulin sensitivity. It's commonly prescribed for type 2 diabetes and insulin resistance conditions. When used together, these medications have several notable interactions: 1. **TSH-Lowering Effect**: Metformin can lower serum thyrotropin (TSH) levels in both hypothyroid patients on levothyroxine therapy and in euthyroid patients. This effect is not due to increased levothyroxine absorption. 2. **Absorption Impact**: Studies found no significant differences in levothyroxine absorption (peak serum total thyroxine (TT4) concentrations and time to peak TT4 concentrations) after metformin use, with only a non-significant trend toward reduced TT4 area under the curve. 3. **Thyroid Volume and Disease Risk**: Patients treated with metformin have been found to have smaller thyroid volumes and a lower risk of goiter, thyroid nodules, and thyroid cancer. Potential mechanisms for metformin's effects on thyroid function include enhancing thyroid hormone effects in the pituitary, activating adenosine monophosphate-activated protein kinase (AMPK), changing the affinity and/or quantity of thyroid hormone receptors, increasing central dopaminergic tone, or inducing activation of the TSH receptor. Additionally, in patients with Hashimoto's thyroiditis, metformin has been shown to reduce thyroglobulin antibodies (TgAb), inflammation in thyroid tissue, and lymphocyte infiltration. It may also modulate Th17 cells, involved in autoimmune responses.