Effect of thyroid hormone on plasma apolipoproteins and apoA- and apoB-containing lipoprotein particles

Apolipoprotein and apoB- and apoA-containing lipoprotein particle concentrations were determined in 10 athyreotic patients 4 weeks after withdrawal of Synthroid replacement therapy [T4, 0.96 +/- 0.66 microgram mL-1; thyroid-stimulating hormone (TSH), 62.7 +/- 22.8 muIU mL-1] and again 4 weeks after reinstitution of treatment.

Thyroid hormone replacement was associated with significant decreases in plasma cholesterol (TC), triglyceride (TG), apolipoprotein B and apolipoprotein C-III (P < 0.01). Both the cholesterol ester-rich LP-B particles and triglyceride-rich LP-Bc particles declined significantly in response to thyroid hormone (LP-B withdrawal 81.6 +/- 24.0 vs. replacement 65.1 +/- 22.0; LP-Bc withdrawal 14.3 +/- 6.0 vs. replacement 10.9 +/- 4.8 mg%, P < 0.01). ApoC-III also decreased in high-density lipoprotein (HDL) (apoC-III-HS), and in very low-density lipoprotein (VLDL) + low-density lipoprotein (LDL) (apoC-III-HP), but this reduction was proportionate so that the apo-C-III-HS/apoC-III-HP ratio, an indirect estimate of the efficiency of lipoprotein lipase (LPL), was unchanged. Apolipoprotein A-I concentrations also decreased significantly (withdrawal 140.7 +/- 27.0 vs. replacement 127.1 +/- 30.0 mg%, P < 0.01) in parallel with the changes in LP-A-I and LP-A-I:A-II particles (LP-A-I withdrawal 35.8 +/- 7.7 vs. replacement 31.5 +/- 6.3; LP-A-I:AII withdrawal 104.9 +/- 20.0 vs. replacement 95.5 +/- 26.0; P < 0.05).

These findings indicate that thyroid hormone influences the transport not only of both TG-rich and cholesterol-rich apoB-containing lipoprotein particles but also of those that contain apoAI.