No matter how strong the anabolic signal is that a muscle cell receives, it cannot grow unless nutrients are available at the site when the message is sent. The rate at which nutrients become available depends on diet, and metabolism. Even if an athlete ingests enough protein, carbs, and fats, it is possible, due to digestive capabilities and metabolic rate, that not enough nutrients will be available at the cell for significant increases in growth.
Thyroid hormones control most of the metabolic rate and therefore the assimilation of protein, carbs, and fats (which greatly influence protein synthesis).
Thus the protein turn over rate is influenced as well. Human Growth Hormone can decrease thyroid hormone activity by inhibiting the conversion of T-4 thyroid hormone into the more active T-3 hormone, so muscle protein synthesis and growth slows down.
A note of interest is the fact that many athletes have shown low levels of phenylalanine, tyrosine (which are amino acids) and iodine when tested. The body uses an enzyme called phenylalanine-hydroxylase to convert phenylalanine into tyrosine. Some athletes suffer from a disease called phenylketonuria (PKU) which is a lack of this conversion enzyme. This leads to excess unconverted phenylalanine, and therefore insufficient available tyrosine for the production of such things as catecholamine neurotransmitters from L-dope to adrenaline. Another problem is that PKU prevents tyrosine from reaching the thyroid gland. Who cares? Well, tyrosine is the amino acid part of thyroxin (T-4) and iodine is the co-factor which allows it to be manufactured.
Low tyrosine, phenylalanine, and/or iodine means low thyroxin T-4 production. This means…low conversion of T-4 to T-3 and slow metabolism, growth decreases or halts, protein synthesis is a joke, and athletes get fat. It also means there is inadequate nutrients at the muscle cell when the anabolic message arrives.
Often supplementing with 1000 mg (1 gram) each of Tyrosine and phenylalanine 3 times daily (In addition to a high protein diet solves the problem of decreased metabolic rate. Amino acid tests such as a QUAAST can find the shortage. PKU sufferers should not take supplemental phenylalanine. Three grams total daily of tyrosine would be a better choice for PKU sufferers.
So now we know that thyroid hormones were actually quite anabolic. However, the difference between anabolic and catabolic was adequate supplies of nutrients and training stimuli. Thyroid hormone levels decide metabolic rate to a great extent and therefore dictated post workout recovery time. The higher the metabolic rate, the quicker recovery… the more calories were necessary.