Objective: Growth hormone (GH) influences thyroid function and anatomy. While goiter is frequent in acromegalic patients, the effects of GH deficiency are difficult to assess, as hypopituitaric subjects who lack GH have often also partial or complete deficit of TSH.

Study Design: We studied thyroid morphology and serum levels of thyroid hormones in adult members of a large Brazilian kindred with untreated isolated GH deficiency (GHD) due to a homozygous mutation in the GHRH receptor gene (GHRHR) (9 men and 15 women; GHD group), and compared them to subjects heterozygous for the same mutation (8 men and 10 women; HET group), and subjects homozygous for the wild-type allele (7 men and 11 women; CO group).

Results: GHD subjects had smaller thyroid volume (TV) than HET and CO. TV of the HET group was intermediate between the GHD and CO groups. When TV was corrected by body surface area, it remained smaller in the GHD and HET than CO group, but the difference between GHD group and HET group disappeared. GHD group had lower serum T3 levels than CO, and higher free T4 (FT4) levels than HET and CO.

Conclusions: Individuals with severe untreated GHD due to a homozygous GHRHR mutation, and heterozygous carriers of the same mutation, have smaller TV than normal subjects, suggesting that GH has a permissive role on the growth of the thyroid gland. In addition, GHD subjects have reduced serum total T3 and increased serum FT4, suggesting a reduction in the function of the deiodinase system.