BACKGROUND
Hypothyroidism is common and can affect all aspects of the body’s functioning. Most hypothyroidism is overt, meaning that the TSH is high and the FT4 level is low. Overt hypothyroidism is almost always treated. Subclinical/mild hypothyroidism is seen where the FT4 is normal and the TSH is high and the benefits of treatment are much less clear. It is well-recognized that overt hypothyroidism is one risk factor for kidney problems. At the same time, the kidney contributes to the regulation of thyroid hormone levels by producing enzymes that convert T4 to the active hormone T3 and by eliminating the thyroid hormones from the body through the urine. The relationship between subclinical hypothyroidism and kidney function has been studied with unclear results.

This large population study evaluated the risk of subclinical hypothyroidism on the development of chronic kidney disease (CKD).

THE FULL ARTICLE TITLE
Kim HJ et al 2023 Subclinical thyroid dysfunction and chronic kidney disease: A nationwide population-based study. BMC Nephrol 24:64. PMID: 36949396.

SUMMARY OF THE STUDY
The Korea National Health and Nutrition Examination Survey (KNHANES; a nationwide, cross-sectional survey) VI (2013–2014) data was analyzed for this study. Included in the analysis were 3257 adult participants who underwent both thyroid and kidney function tests. The participants had normal FT4 levels, no history of thyroid disease, no other chronic diseases and were not on any medications that could influence thyroid function. CKD was defined as a decreased estimated glomerular filtration rate, the main measurement of kidney function.

Of the 3257 participants, 54.1% were men with an average age of 44.1 years and 6.7% had CKD. Participants with CKD were significantly older (average age, 54.7 vs. 43.3 years), had relatively lower income/education and higher measurements of body-mass indexes, systolic and diastolic blood pressures, total cholesterol, triglycerides and fasting glucose. Comparing the entire group participants, there was no difference in serum TSH levels. However, examining the group of individuals with subclinical hypothyroidism demonstrated a ~2-fold greater risk of CKD than those with normal TSH levels. Even with adjustments made for multiple variables including sex, age, household income, education, smoking, alcohol use, walking activity, obesity, high blood pressure and diabetes, individuals with subclinical hypothyroidism continued to have a >2-fold risk factor for CKD.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This broad, population-based survey from South Korea suggests that subclinical hypothyroidism independently predicted and was associated with increased odds of CKD. What is unclear is whether treatment of subclinical hypothyroidism affected this risk and should be the basis of future studies. However, this data shows that it is important to follow kidney function in patients with subclinical hypothyroidism.

— Alan P. Farwell, MD

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