Thyroid peroxidase activity is inhibited by amino acids

Also see:
Protective Glycine
Gelatin, Glycine, and Metabolism
Gelatin > Whey
Serotonin, Fatigue, Training, and Performance
Carbohydrate Lowers Serotonin from Exercise

“Muscle catabolism also releases a large amount of cysteine, and cysteine, methionine, and tryptophan suppress thyroid function (Carvalho, et al., 2000).” -Ray Peat, PhD

“Muscle protein is very rich in tryptophan and cysteine, and these amino acids suppress the thyroid gland’s function, and are potentially toxic to nerves, especially in the presence of cortisol and hypoglycemia. Tryptophan is turned into serotonin, which promotes lipid peroxidation, blood clotting, and certain patterns of nerve activity. Serotonin can suppress mitochondrial respiration, and along with the reduced body temperature that it produces, a pattern or torpor or helplessness tends to be produced.” -Ray Peat, PhD

“The selection of amino proteins should minimize the amino acids tryptophan (which is the precursor to serotonin) and cysteine (which like tryptophan, suppresses thyroid function), by including gelatin and fruits. Gelatin is 22% glycine, which protects the lungs and other organs against toxins and inflammatory agents, and many fruits are also “deficient” in tryptophan and cysteine.” -Ray Peat, PhD

Braz J Med Biol Res. 2000 Mar;33(3):355-61.
Thyroid peroxidase activity is inhibited by amino acids.
Carvalho DP, Ferreira AC, Coelho SM, Moraes JM, Camacho MA, Rosenthal D.
Normal in vitro thyroid peroxidase (TPO) iodide oxidation activity was completely inhibited by a hydrolyzed TPO preparation (0.15 mg/ml) or hydrolyzed bovine serum albumin (BSA, 0.2 mg/ml). A pancreatic hydrolysate of casein (trypticase peptone, 0.1 mg/ml) and some amino acids (cysteine, tryptophan and methionine, 50 microM each) also inhibited the TPO iodide oxidation reaction completely, whereas casamino acids (0.1 mg/ml), and tyrosine, phenylalanine and histidine (50 microM each) inhibited the TPO reaction by 54% or less. A pancreatic digest of gelatin (0.1 mg/ml) or any other amino acid (50 microM) tested did not significantly decrease TPO activity. The amino acids that impair iodide oxidation also inhibit the TPO albumin iodination activity. The inhibitory amino acids contain side chains with either sulfur atoms (cysteine and methionine) or aromatic rings (tyrosine, tryptophan, histidine and phenylalanine). Among the amino acids tested, only cysteine affected the TPO guaiacol oxidation reaction, producing a transient inhibition at 25 or 50 microM. The iodide oxidation inhibitory activity of cysteine, methionine and tryptophan was reversed by increasing iodide concentrations from 12 to 18 mM, while no such effect was observed when the cofactor (H2O2) concentration was increased. The inhibitory substances might interfere with the enzyme activity by competing with its normal substrates for their binding sites, binding to the free substrates or reducing their oxidized form.