Table 3.

Reports on seasonal variation in thyroid function in the last two decades

	Country	Latitude	Number examined	Ethnicity	TSH levels	FT3 levels	FT4 levels	Diseases and blood withdrawal time	Reference
1	Israel	31°N	46 million persons	Broad ethnic representation	Major peak in August and minor peak in winter	High in winter	High in late autumn		Tendler et al. (2021) [5]
2	Italy (Modena)	44°N	1,506,495 data points	n/a	High in summer and winter	No change	No change		Santi et al. (2019) [6]
3	China (Peking)	39°N	6,524 subjects	n/a	High in winter, low in summer	No change	No change	All subjects were >65 years old.	Wang et al. (2019) [7]
4	Iraq (Sulaymaniyah)	35°N	152 healthy volunteers, 25 SCH subjects	n/a	No change	High in winter	No change	Samples were taken between 9:00 and 11:00 am after an overnight (12 hour) fast.	Mahwi et al. (2019) [8]
5	China (Peking)	39°N	79,570 data points	n/a	High in winter, low in summer	n/a	n/a		Wang et al. (2018) [9]
6	Japan (Tokyo)	36°N	135,417 patients	n/a	High in winter, low in summer	High in winter, low in summer	High in summer, low in autumn	All subjects had thyroid disorder. Samples were taken between 8:00 am and 5:00 pm.	Yoshihara et al. (2018) [10]
7	UK, Japan, Belgium	51°N, 36°N, 50°N	The daily medians are calculated from at least 500 patients.	n/a	High in winter, low in summer	n/a	n/a		De Grande et al. (2017) [11]
8	United States (Salt Lake City)	40°N	324,750 outpatients	Caucasian 78.4%, Native American 0.4%, Pacific Islander 0.7%, African American 0.8%, Asian 1.4%, Other	Low in August, high in December	n/a	No change	No diagnosis of thyroid disease.	Ehrenkranz et al. (2015) [12]
9	Russia (Siberia)	62°N	35 men, 59 women	Yakut (Sakha) adults	No change	Low in winter	Low in winter	Samples were taken in July/August and January in the morning after overnight fasting.	Leonard et al. (2014) [13]
10	Russia(Siberia)	62°N	51 men, 83 women	Yakut (Sakha) adults	High in winter	Low in winter	Low in winter	Samples were taken in July/August and January in the morning after overnight fasting.	Levy et al. (2013) [14]
11	Korea (Seoul)	37°N	28,096 subjects	n/a	High in winter and spring, low in summer and autumn	n/a	n/a	Samples were taken after 12-hour overnight fasting.	Kim et al. (2013) [15]
12	Finland (Northern Finland)	67-68°N	20 men	Caucasian	High in winter	High in summer	No change	Samples were taken between 9:00 and 11:00 am.	Hassi et al. (2001) [16]

Abbreviations: FT3, free triiodothyronine; FT4, free thyroxine; n/a, not available; SCH, subclinical hypothyroidism; TSH, thyrotropin.