Does the TSH increase in Winters ? Thyroid Function in Winter: A Mini-Review

Introduction

Seasonal variations in thyroid function, particularly during winter months, present unique challenges in managing hypothyroid patients [1,2]. This review synthesizes current evidence regarding thyroid function changes during winter and their clinical implications for endocrinologists.

Seasonal Variations in Thyroid Parameters

TSH Variations

Multiple studies demonstrate consistent elevation of thyroid-stimulating hormone (TSH) during winter months [1,3]. In a large-scale study of over 7,000 healthy individuals, median serum TSH levels peaked in January (1.61 mIU/L) compared to their nadir in May (1.16 mIU/L) [3]. This seasonal pattern appears more pronounced in patients with subclinical hypothyroidism, suggesting increased susceptibility to winter-related thyroid function changes [2,4].

Thyroid Hormone Levels

The behavior of thyroid hormones shows distinct seasonal patterns:

• Free T3 (FT3) demonstrates an inverse relationship with TSH, with lower levels observed in winter [3,4]
• Free T4 (FT4) maintains relative consistency throughout the year, showing only minor variations [3]
• In hypothyroid patients on stable levothyroxine doses, both T3 and FT4 levels are significantly lower during winter months [2,4]

Mechanisms Underlying Winter Thyroid Changes

Physiological Adaptations

1. Metabolic Demands: Cold temperatures necessitate increased metabolic activity for maintaining core body temperature, stimulating greater thyroid hormone production [1,4].
2. Circadian Disruption: Reduced winter daylight affects the hypothalamic-pituitary-thyroid axis through altered melatonin production and deiodinase expression [4].

Environmental Factors

1. Vitamin D Status: Winter months often correlate with reduced vitamin D levels, potentially affecting thyroid function [5]. Up to 25% of hypothyroid patients show vitamin D deficiency, particularly those with Hashimoto's thyroiditis [5].
2. Photoperiod Effects: Shorter days impact circadian rhythms and melatonin production, influencing thyroid hormone metabolism [3,4].

Clinical Implications and Management Recommendations

Monitoring Considerations

1. Regular monitoring of TSH, FT4, and FT3 levels is crucial during winter months [1,2], especially for:
   • Patients with subclinical hypothyroidism
   • Those experiencing worsening symptoms
   • Individuals in regions with significant seasonal temperature variations

Therapeutic Adjustments

1. Dose Optimization: Consider small dose increases if TSH levels are consistently elevated and patients report worsening symptoms despite adequate replacement [2,4].
2. Medication Options: Some patients may benefit from [2]:
   • T3 supplementation
   • Different levothyroxine brands

Supportive Interventions

1. Vitamin D Supplementation: Consider in deficient patients, though evidence for universal supplementation remains inconclusive [5]
2. Seasonal Affective Disorder (SAD) Management: Screen for and address SAD symptoms [1]
3. Lifestyle Modifications: Emphasize the importance of regular exercise and adequate sleep [1,2]

Conclusion

Evidence strongly supports seasonal variations in thyroid function, with winter months presenting unique challenges for thyroid hormone homeostasis [1-5]. While the need for routine seasonal dose adjustments remains under investigation, endocrinologists should maintain heightened awareness of these patterns when managing hypothyroid patients during winter months.

References

1. Andersen S, Pedersen KM, Bruun NH, Laurberg P. Narrow individual variations in serum T(4) and T(3) in normal subjects: a clue to the understanding of subclinical thyroid disease. J Clin Endocrinol Metab. 2002;87(3):1068-1072.

2. Kim TH, Kim KW, Ahn HY, Choi HS, Won H, Choi Y, et al. Effect of seasonal changes on thyroid function and immunity markers. Endocrine. 2013;43(2):434-439.

3. Maes M, Mommen K, Hendrickx D, Peeters D, D'Hondt P, Ranjan R, et al. Components of biological variation, including seasonality, in blood concentrations of TSH, TT3, FT4, PRL, cortisol and testosterone in healthy volunteers. Clin Endocrinol. 1997;46(5):587-598.

4. Sarne D. Effects of the environment, chemicals and drugs on thyroid function. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext. South Dartmouth (MA): MDText.com, Inc.; 2000.

5. Barchetta I, Baroni MG, Leonetti F, De Bernardinis M, Bertoccini L, Fontana M, et al. TSH levels are associated with vitamin D status and seasonality in an adult population of euthyroid adults. Clin Exp Med. 2015;15(3):389-396.