About 23 million people in the United States take a drug called levothyroxine (marketed as Synthroid) for hypothyroidism, and new research has suggested that the vast majority of them – as many as 90 percent – are taking the drug unnecessarily. Thyroid-stimulating hormone (TSH) and free thyroxine levels are measured during testing to determine whether a person has hypothyroidism, which is characterized by unusually low thyroid gland function (Free T4). This study has indicated that there are seasonal variations to these hormones, which is not accounted for in laboratory testing, and is probably leading to the misdiagnosis of hypothyroidism and a huge number of unneeded levothyroxine prescriptions. The findings have been reported in Clinical Chemistry.