Aim: Thyroid hormone affects whole-body metabolisms, such as lipids and glucose metabolism. Accurate evaluation of thyroid function is therefore important. In this study, seasonal changes in thyroid function in over 14,000 normal subjects were investigated, and the effects of gender and age, smoking were also examined.
Subjects and methods: The subjects were participants of medical checkups at Hidaka Hospital from 2006 to 2013. Exclusion criteria were a past history of thyroid disease, steroid use, renal failure and liver cirrhosis. We examined 8,489 men and 5,534 women whose blood TSH and FT4 levels were within the reference values for monthly changes in thyroid function. All blood TSH and FT4 levels were measured at 8 ~ 9:00 AM after over night fasting.
Results: The average age were 49 ± 10 years old in men and 48 ± 10 years old in women. In men the highest median TSH level was 1.6 μU/ml in January, and the lowest was 1.3 μU/ml (p <0.01) in June, July, August and September. These values were significantly different from the yearly median value of 1.4 μU/ml in April and November. On the other hand, the FT4 level of men was significantly lower in August than in the other months except for February. The extents of seasonal changes in TSH levels were reduced in women and in elderly men. Furthermore, in men the median blood TSH level of 2,262 smokers was lower than that of 4,709 non-smokers (median: smoker, 1.2 vs. non-smoker, 1.5), and the seasonal changes in TSH and FT4 levels were reduced in smokers.
Conclusions: This is the first report in large healthy cohort demonstrating that there are significant differences on seasonal changes in thyroid function by gender, age and smoking. These need to be taken into account for accurate evaluation of thyroid function.