After bilio-pancreatic bypass surgery in severely obese adults, levels of the active thyroid hormone FT3 drop by 10.5% within three months and stay 11.2% lower for up to three and a half years, even...
Mechanism
Synthesis from 1 study
After surgery that stops fat from being digested, the body can't make enough active thyroid hormone because it lacks the fats needed for the conversion process. This low level of active hormone stays low for years, even after weight loss stops, because the problem is not weight — it's the lack of...
Most probable mechanism
After surgery that prevents proper fat digestion, the body lacks the fats needed to activate enzymes that convert thyroid hormone into its active form. This causes a lasting drop in active thyroid hormone, even after weight stops changing.
Bilio-pancreatic bypass diverts bile and pancreatic enzymes away from the duodenum, preventing emulsification and digestion of dietary fats.
Reduced fat absorption limits the availability of fatty acids and cholesterol, which are required cofactors for deiodinase type 1 enzyme activity in the liver and kidneys.
Deiodinase type 1 activity decreases, reducing the conversion of thyroxine (T4) into the biologically active triiodothyronine (T3).
Circulating free triiodothyronine (FT3) levels fall and remain suppressed long-term, even after body weight and composition stabilize.
Less supported by current evidence, but not ruled out
When the body burns more protein for energy after surgery, it sends signals that adjust the pituitary gland's target level for thyroid hormone, keeping TSH normal even when active thyroid hormone is low.
Post-surgical nutrient restriction shifts energy metabolism toward increased protein breakdown and oxidation.
Elevated amino acid catabolism generates metabolic signals that alter hypothalamic TRH release or pituitary thyrotroph sensitivity.
The pituitary gland maintains normal TSH secretion despite low FT3 and normal FT4, indicating a recalibrated set point for thyroid hormone regulation based on substrate availability.
Evidence from Studies
Supporting (1)
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