Thyroid function tests are some of the most commonly ordered tests in the USA (1). Meanwhile, levothyroxine is also the most prescribed drug in the USA, with an estimated 23 million people who have an active prescription (2). However, studies have raised concerns about levothyroxine overuse, with some suggesting that approximately 90% of prescriptions are inappropriate because they were given to patients with mild subclinical hypothyroidism or who were euthyroid without significant changes in patterns over time or any improvement in health-related quality of life (2). The recent publication by Yamada et al. (3) in the Journal of the Endocrine Society describes the effect of seasonal variation in thyroid function results in >7000 healthy Japanese participants and directly links that to a fraction of subclinical dysfunction diagnoses (subclinical hyperthyroidism and subclinical hypothyroidism). This study showed that thyrotropin-stimulating hormone (TSH) varies widely throughout the seasons, peaking in the winter months (January–February) with its nadir in the summer months (June–August). However, free thyroxine (Free T4) was relatively stable. These seasonal changes in healthy individuals, with TSH going up while Free T4 remains stable, are not captured by our reference intervals and may lead to false diagnoses of subclinical hypothyroidism and unnecessary prescriptions of levothyroxine to euthyroid individuals. This is clearly demonstrated in Yamada et al.’s study, in which the healthy population’s upper 97.5th percentile ranges for TSH exceeded the manufacturer-recommended limit for the months of January and February, leading to higher rates of subclinical hypothyroidism diagnoses. In addition to seasonal variability, there are other factors associated with TSH variation that may contribute to misdiagnoses, including: age, sex, time-of-day, and transient elevation during nonthyroidal illness (4).