Relationships between thyroid function and lipid status or insulin resistance in a pediatric population

In adults without thyroid disease, increasing levels of thyroid-stimulating hormone (TSH) within the range of that considered normal have been shown to be associated with increases in total cholesterol, low-density lipoprotein cholesterol, non-high-density lipoprotein cholesterol, and triglycerides, and with decreases in high-density lipoprotein cholesterol. Serum TSH has also been found to be positively associated with fasting and postload insulin concentrations and negatively associated with insulin sensitivity in euthyroid adults. We hypothesized that such relationships also exist in euthyroid children and adolescents.

This was a retrospective record review of pediatric outpatients (ages 2-18 years) having measurements of TSH or free thyroxine (T4) and a concurrent lipid panel, fasting glucose, or fasting insulin. Pearson correlation coefficient was used to estimate the correlation between TSH or free T4 and logarithmic transformed lipid, plasma glucose, or insulin levels. Lipid levels, fasting plasma glucose, insulin, and homeostasis model assessment (HOMA) were also compared between subjects with TSH levels in the high normal range (2.5-5 mIU/L) and those with TSH in the low normal range (0.3-2.4 mIU/L).

TSH levels were positively correlated with triglyceride levels (r = 0.10, p = 0.001). Conversely, free T4 levels were inversely correlated with triglyceride levels (r = -0.10, p = 0.011). TSH levels were also positively correlated with fasting insulin (r = 0.26, p = 0.002) and with HOMA (r = 0.27, p = 0.001). These associations remained significant after adjustment for age, gender, and body mass index z-score. Children who had TSH levels between 2.5 and 5.0 mIU/L had higher triglycerides (p = 0.003), insulin levels (p = 0.040), and HOMA (p = 0.021) than those having TSH values between 0.3 and 2.4 mIU/L.

In euthyroid children without a history of hypo- or hyperthyroidism, increasing levels of TSH and decreasing levels of free T4 are associated with higher triglyceride levels and elevated markers of insulin resistance. Whether these findings carry implications regarding optimal TSH levels in children at increased risk for cardiovascular disease awaits further study.