We aimed to evaluate catch-up growth in children with severe Hashimoto's hypothyroidism (HH) after thyroid hormone replacement therapy (HRT).

A multicenter retrospective study was conducted including children referred for growth slowdown that led to the diagnosis of HH between 1998 and 2017.

A total of 29 patients were included, with a median age of 9.7 years (13–172 months). Median height at diagnosis was -2.7 [-4.6; -0.1] standard deviation score (SDS), with a height loss of 2.5 [0.7; 5.4] SDS compared to height before growth deflection (p<0.0001). At diagnosis, the median TSH level was 819.5 mIU/L [100; 1844], the median FT4 level was 0 pmol/L [undetectable; 5.4], and the median anti-thyroperoxidase antibody level was 1601 UI/L [47; 25,500].

In the 20 patients treated only with HRT, there were significant differences between height at diagnosis and height at 1 year (n = 19, p<0.0001), 2 years (n = 13, p = 0.0005), 3 years (n = 9, p = 0.0039), 4 years (n = 10, p = 0.0078), and 5 years (n = 10, p = 0.0018) of treatment but not in the case of final height (n = 6, p = 0.0625). Median final height was -1.4 [-2.7; 1,5] SDS (n = 6), with a significant difference between height loss at diagnosis and total catch-up growth (p = 0.003).

The other nine patients were also given growth hormone (GH). They were smaller at diagnosis (p = 0.01); however, there was no difference in final height between those two groups (p = 0.68).

Severe HH can lead to a major height deficit, and catch-up growth seems to be insufficient after treatment with HRT alone. In the most severe cases, administration of GH may enhance this catch-up.