Scant data exist on the longitudinal association between thyroid
function and lipid concentrations. We investigated associations of TSH
and lipid concentrations cross-sectionally and longitudinally in a
nationwide population sample.

A total of 5205 randomly sampled participants representative of Finns
aged ≥30 years were examined in 2000-2001 and included in
cross-sectional analyses. A total of 2486 were re-examined 11 years
later and included in longitudinal analyses. With linear regression
models adjusted for age, gender, smoking and body mass index, we
assessed the associations of baseline TSH and TSH categories (low,
reference range and high) with total, high-density lipoprotein (HDL) and
low-density lipoprotein (LDL) cholesterol; apolipoprotein A1 and B; and
triglycerides at baseline and follow-up.

At baseline, higher TSH associated with higher total cholesterol (β =
0·025, standard error [SE] = 0·007, P < 0·001), LDL cholesterol (β =
0·020, SE = 0·007, P = 0·002), apolipoprotein B (β = 0·006, SE = 0·002, P
< 0·001) and log triglycerides (β = 0·008, SE = 0·003, P = 0·004),
but not with other lipid outcomes. Higher baseline TSH associated with
higher total cholesterol (β = 0·056, SE = 0·026, P = 0·033), LDL
cholesterol (β = 0·057, SE = 0·023, P = 0·015) and apolipoprotein B (β =
0·012, SE = 0·006, P = 0·028) at follow-up in women, but not with any
lipid outcomes in men. Participants with high TSH at baseline had a 0·22
mmol/l (95% confidence interval 0·02-0·41 mmol/l) higher LDL
cholesterol at follow-up (P = 0·028) than participants with TSH in the
reference range (0·4-3·4 mU/l). However, exclusion of participants with
high-risk baseline lipid values rendered these positive longitudinal
associations nonsignificant (P ≥ 0·098).

We could confirm a modest association between higher TSH and an adverse
lipid profile cross-sectionally but not indisputably longitudinally.