Findings from a community-based, cross-sectional study showed that lifestyle influences thyroid function, with poor sleep quality (P =.012) and iodine excess (P =.048) increasing the risk of developing subclinical hypothyroidism. These finding were published in BMC Endocrine Disorders.

Exercise, iodine excess, and staying up late on weekends were found to influence thyroid secretory capacity. Exercise was also an influencing factor in thyroid-stimulating hormone (TSH) levels and thyrotropin resistance. Smoking and iodine excess affected pituitary thyroid-stimulating function.

This study was conducted in 2016 and compared 159 participants from the Fujian Province of China who had subclinical hypothyroidism (81 men and 78 women) with 159 euthyroid control group participants (87 men and 72 women). The investigators collected general lifestyle information regarding the participants’ sleep quality via the Pittsburgh Sleep Quality Index questionnaire, as well as sleep, exercise, diet, and smoking habits via questionnaires.


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The researchers collected fasting venous blood samples to measure TSH, free thyroxine (FT4), free tri-iodothyronine (FT3), thyroid peroxidase antibody, and thyroid globulin antibody. The investigators obtained urine samples to determine urine iodine concentration.

They then calculated thyroid homeostasis parameter thyroid’s secretory capacity, Jostel’s TSH Index, and thyrotroph T4 sensitivity index, also known as thyrotroph thyroxine resistance index, and assessed associations using logistic regression and multiple linear regression.

Hypothyroidism tended to increase in severity with smoking, particularly among women with subclinical hypothyroidism, who demonstrated higher TSH levels than their nonsmoking counterparts. Thus, smoking influences pituitary thyroid-promoting function. The researchers found that those in the subclinical hypothyroidism group had higher rates of thyroid antibodies than those in the control group. This finding supports the observation that thyroid autoimmunity is the primary cause of subclinical hypothyroidism.

The study authors suggest that “about 5% of [case of subclinical hypothyroidism] may gradually progress to clinical hypothyroidism without treatment….Therefore, [subclinical hypothyroidism] needs to be paid enough attention and corresponding measures should be taken to reduce its harm.” Such harm includes cardiovascular disease risk, insulin resistance, and metabolic syndrome, all of which increased TSH may influence.

TSH, the primary hormone affected by circadian rhythms, may explain the correlation between subclinical hypothyroidism and sleep quality affecting TSH secretion. Regular exercise of moderate to high intensity correlates with lower TSH levels, improved thyroid secretion, and lower thyrotropin resistance.

Iodine excess enhances thyroid secretion and the pituitary’s ability to promote thyroid secretion. This study confirmed that clinicians should assess the relationship between thyroid secretion capacity and iodine nutrition status via urine iodine concentration instead of the frequency of intake of iodine-rich foods.

The study authors assert that clinicians may more accurately assess the functional status of the hypothalamus pituitary thyroid axis using thyroid homeostasis parameters of thyroid secretion capacity rather than the typical thyroid function panel measuring TSH and T4, which may not reflect the dynamic changes of the hypothalamus pituitary thyroid axis correctly.

A limitation of this study was its cross-sectional nature, which muddied the cause-and-effect projection between lifestyle and subclinical hypothyroidism. Other limitations included the small sample size, the lack of subject diversity, the lack of subject location, the overall adequate iodine nutrition, and probable selection bias.

Reference

Wu K, Zhou Y, Ke S, et al. Lifestyle is associated with thyroid function in subclinical hypothyroidism: a cross-sectional study. BMC Endocr Disord. 2021;21(1):112. doi:10.1186/s12902-021-00772-z