Resting energy expenditure and low-density lipoproteins serum concentration in adults with mild subclinical hypothyroidism

The high prevalence of subclinical hypothyroidism in Chile may be due to the low normal upper limit of serum thyroid-stimulating hormone (TSH). People with TSH slightly higher than the upper limit may be metabolically similar to healthy people. Thyroid action markers (resting energy expenditure [REE] and low-density lipoprotein [LDL]) were compared in adults with mild subclinical hypothyroidism and with normal thyroid function with or without levothyroxine treatment. REE, lipid and thyroid profile were measured in healthy people with normal thyroid function (TSH ≥0,4-<4,5 μUI/ml (n=91); with mild subclinical hypothyroidism (TSH ≥4,5-≤6 μUI/ml; n=5); and with clinical hypothyroidism treated with levothyroxine and normal TSH (n=13). LDL was analyzed in 838 healthy people with normal thyroid function and 89 with mild subclinical hypothyroidism from the 2016/17 National Health Survey (NHS). REE, adjusted for weight, sex and age, was similar between the groups (p=0,71). LDL was similar between people with normal thyroid function and mild subclinical hypothyroidism (91±24 vs. 101±17 mg/dl; p=0,67), and lower in treated hypothyroidism (64±22 mg/dl; p<0,01). LDL was not associated with TSH but was inversely with FT4 in women (r=-0,33; p=0,02; n=53). In the NHS, both groups had similar serum LDL (p=0,34), which was inversely associated with FT4 in women (r=-0,12; p=0,01; n=569), but not with TSH. Healthy people with normal thyroid function and mild subclinical hypothyroidism have similar REE and LDL. These results support the idea of redefining the normal upper limit of TSH.