The interaction between the GH-IGF-I axis and thyroid hormone metabolism is complex and not fully understood. T₄ stimulates IGF-I activity in animals in the absence of GH. On the other hand, GH replacement therapy results in an increase in serum T₃ and a decrease in T₄ and rT₃ levels, suggesting a stimulation of type I deiodinase (D1) activity. Recently, we demonstrated the association of two polymorphisms in D1 (D1a-C/T; T = 34%, and D1b-A/G; G = 10%) with serum iodothyronine levels. Haplotype alleles were constructed, suggesting a lower activity of the D1 haplotype 2 allele (aT-bA) and a higher activity of the haplotype allele 3 (aC-bG). In this study, we investigated whether genetic variations in D1 are associated with the IGF-I system.

In 156 blood donors and 350 elderly men, the association of the D1 haplotype alleles with circulating IGF-I and free IGF-I levels was studied. In addition, potential associations with muscle strength and body composition were investigated in the elderly population. Finally, the relation between serum iodothyronine levels and IGF-I levels was studied.

In blood donors, haplotype allele 2 was associated with higher levels of free IGF-I (302.9 ± 22.9 vs. 376.3 ± 19.1 pg/ml, P = 0.02). In elderly men, haplotype allele 2 also showed an allele dose increase in free IGF-I levels (P_trend = 0.01) and an allele dose decrease in serum T₃ levels (P_trend = 0.01), independent of age. Carriers of the D1a-T variant also had a higher isometric grip strength (P = 0.047) and maximum leg extensor strength (P = 0.07) as well as a higher lean body mass (P = 0.03).

In blood donors, T₄ and free T₄ were negatively correlated with total IGF-I levels (R = −0.18, P = 0.03 and R = −0.24, P = 0.003), whereas T₃ to T₄ and T₃ to reverse T₃ ratios were positively correlated with total IGF-I (R = 0.31, P < 0.001 and R = 0.18, P = 0.03). Free IGF-I showed a negative correlation with T₄ (R = −0.26, P = 0.001) and T₄-binding globulin (R = −0.31, P < 0.001) and a positive correlation with T₃ to T₄ ratio (R = 0.21, P = 0.01).

In conclusion, a polymorphism that results in a decreased D1 activity is associated with an increase in free IGF-I levels. The pathophysiological significance of this association with IGF-I is supported by an increased muscle strength and muscle mass in carriers of the D1 haplotype 2 allele in a population of elderly men. The association of D1 haplotype allele 2 with serum T₃ levels in the elderly population suggests a relative increase in its contribution to circulating T₃ in old age.