Here's some info I pulled from my notes on t3's effects on Growth Hormone.

The regulation of growth hormone (GH) by thyroid hormones (TH) has been shown to present species variation. We investigated the regulation of GH in the eel, representative of an ancient group of teleosts. In vivo administration of triiodothyronine (T3) or thyroxine (T4) significantly reduced pituitary and serum GH levels, as measured by homologous RIA. In order to investigate the ability of TH to regulate GH production directly at the pituitary level, we used a long term, serum-free primary culture of eel pituitary cells. Both T3 and T4 inhibited GH release in a concentration-dependent manner, producing up to 50% of inhibition at 10 nM, with an ED50 of<0.2 nM, within the range of their physiological circulating levels. Other hormones also acting via the nuclear receptor superfamily, such as sex steroids (testosterone, estradiol and progesterone) and corticosteroid (cortisol), had no effect on GH release in vitro, underlining the specificity of TH regulatory effect on GH. Measurement of both GH release and cellular content for calculation of GH production in vitro indicated that TH not only inhibited GH release but also GH synthesis. Dot blot assay of GH messenger RNA (mRNA) using a homologous eel cDNA probe showed a decrease in GH mRNA levels in cells cultured in presence of T3, as compared with control cells. This demonstrated that the inhibition of T3 on GH synthesis was mediated by a decrease in GH mRNA steady levels. In conclusion, we demonstrate inhibitory regulation of eel GH synthesis and release by TH, exerted directly at the pituitary level. These data contrast with the rat, where TH are known to have a stimulatory effect and suggest that the pattern observed here in an early vertebrate and also found in birds, reptiles and some mammals including humans, may represent an ancestral and more generalized vertebrate pattern of TH regulation of pituitary GH.