Effects of Thyroxine or Triiodothyronine on the Chick Thyroid in the Presence or Absence of Exogenous TSH

Abstract

Both thyroxin or triiodothyronine inhibited the expected response to thyroid stimulating hormone (TSH) of an increase in thyroid wt. and thyroid cell height in the 2.5-day White Leghorn cockerel. Neither thyroxin nor triiodothyronine affected thyroid wt. or cell height in birds that received no TSH. This was interpreted as evidence that thyroxin or triiodothyronine acts directly on the thyroid gland to inhibit its response to TSH. When I131 uptake was studied, neither thyroxin nor triiodothyronine interfered with uptake when given with TSH. These thyroid hormones suppressed radioiodine uptake in birds that received no TSH. This was interpreted as evidence that the pituitary secretes minute amts. of TSH which are sufficient to effect I131 collection, yet do not produce discernible changes in thyroid wt. or cell height, and that this pituitary TSH secretion can be inhibited by either thyroxin or triiodothyronine.