Polly: In many cases of autism, there is a strong serotonin influence. A heightened influence of serotonin can cause central hypothyroidism, which occurs in about 30% of the autistic. Serotonin can also interfere with intestinal motility, pancreatic function, and mitochondrial energy production. It can even make a person more susceptible to seizures.
Platelet serotonin levels are usually high in autism, but they can also be abnormally low in autism.  (A similar pattern of plasma rich platelet readings of serotonin occurs in fibromyalgia. Like in autism, there is a very high incidence of central hypothyroidism.) In autism, there is altered serotonin synthesis in a brain pathway important for language production and sensory integration.  In infantile autism, the platelets were found to be leaking serotonin.  (This leaking of serotonin is also suspected in fibromyalgia.) Serotonin that has leaked out of cells is called unbound or free serotonin. Unbound or free serotonin would cause the body to act as if it has too much serotonin. If free serotonin is high, what could be causing this? Here are some possible reasons for excessive free serotonin.
1. Lipopolysaccharide (LPS). This is a toxin that is found in the shell of gram-negative intestinal bacteria. LPS can release serotonin from mast cells. LPS can also increase platelet aggregation and this leads to increased release of serotonin from the platelets.  Hence LPS can increase free serotonin levels.
2. Intestinal Inflammation. This causes the intestinal enterochromaffin cells to dump serotonin. Some of this ends up in the general circulation.
3. Low Magnesium. This increases the likelihood that platelets and mast cells will release their serotonin and histamine.
4. Stress Plus Polyunsaturated Oils. Stress, when combined with excess polyunsaturated oils in the diet, will cause serotonin to leak from cells.
This is Dr. Raymond Peat’s explanation of the relationship between stress, fats, and serotonin leaking from cells:
Stress also liberates free fatty acids from storage, and these fatty acids increase the uptake of tryptophan into the brain, increasing the formation of serotonin. … Serotonin liberates polyunsaturated fats, and these in turn liberate serotonin from cells such as the platelets, and liberates tryptophan from serum albumin, increasing its uptake and the formation of serotonin in the brain. Saturated fats don’t liberate serotonin … 
Therefore, it seems prudent to eliminate most of the polyunsaturated oils in the diet Corn, safflower, cottonseed, peanut, canola, soy, fish and flax oils are all high in polyunsaturated fatty acids. Notice that fish and flax oils are in this list of polyunsaturated oils. Even though there is some benefit to limited use of fish and flax oils, too much can be quite harmful. In my opinion, one should limit their fats/oils to mainly butter, olive and coconut.
Natural factors that will help keep serotonin levels under control are thyroid, protein, magnesium, carbon dioxide, decreased tryptophan consumption, vitamin B1, progesterone and exposure to light.  Also, the amino acid glycine may counter some of the effects of excess serotonin. Compare this list of harmful effects of excess serotonin with this list of protective properties of glycine. Raymond Peat, PhD states:
“Serotonin excess produces a broad range of harmful effects: Cancer, inflammation, fibrosis, neurological damage, shock, bronchoconstriction, and hypertension, for example. Increased serotonin impairs learning, serotonin antagonists improve it. The simplest, nonessential, amino acid, glycine, has been found to protect against carcinogenesis, inflammation, fibrosis, neurological damage, shock, asthma, and hypertension. Increased glycine improves learning (Handlemann, et al., 1989; File, et al., 1999), glycine antagonists usually impair it.” 
Although the benefits of a glycine supplement are impressive, please be careful. Glycine can feed some bacteria in preference to others. This shift in population may or may not be good. If you wish to purchase some glycine, a bottle of just glycine powder will be cheaper than encapsulated glycine. Glycine tastes sweet and not bitter, so most people won’t mind the taste of it.
Since platelet serotonin is often high in autism, people have thought that lowering serotonin levels might be helpful. There was a study where tryptophan was lowered very quickly with an amino acid drink. That turned out to be a mistake. It made many of the kids worse. I suspect the failure of this approach may have been due in part to the very abrupt lowering the serotonin or perhaps other effects of the amino acid drink itself. For instance, a quick lowering of serotonin could cause a migraine.
If such an experiment is tried again, tryptophan should be lowered gradually with diet. Also, thyroid medication may have to be lowered if tryptophan is restricted in the diet. (Tryptophan restriction increases T3 thyroid. ) The age of the person must be taken into consideration too, since tryptophan is needed for a child to grow. However, as an added bonus, with less tryptophan, there would be less formation of IAG, which can break down the blood-brain barrier. (IAG is Indolyl Acryloyl Glycine. It is formed by certain bacteria acting on tryptophan.)
Hope for Autism through Nutrition The Health Forum—Book 5