Brief Answer:
You might have irritable bowel syndrome

Detailed Answer:
Hello,
1) You mentioned that previous CT scan of abdomen was normal + intolerance to fatty food + loss of weight. These are consistent with irritable bowel syndrome
2) This is physiological condition so it can not be detected in CT scan. In this condition bowel movement becomes abnormal (mostly diarrhea but sometimes constipation also) due to exposure of intestine to particular type of food. This particular food type varies from person to person.
3) In this condition absorption of nutrient from diet will be deranged due to fasten bowel movement and thus weight loss occur due to malabsorption.
4) Sometimes inner layer of intestine will be inflamed (redness and swelling) in this condition which can be confirmed in biopsy taken with help of endoscopy.
5) You need supplementation of digestive enzymes which is available as medication (once disease is confirmed) and try to avoid particular type of diet (fatty food in your case) which aggravates condition.
6) Another possibility is chronic pancreatitis in which you develop intolerance to fatty food due to insufficient pancreatic enzymes for digestion of fatty food. But this condition is associated with alcoholism and it has changes in structure of pancreas on CT scan. Thus possibility of having this condition is less for you based on your CT scan report.
7) You should not repeat thyroid test again for these symptoms.

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