Will low TH positively feed back to increase TSH levels?

[theWUbear]

---

Members don't see this ad.

Here's a basic question regarding hormone regulation, covered on the mcat.

If a patient has iodine deficiency, he cannot produce sufficient thyroid hormones.

Conventional logic regarding normal feedback loops would have it that this means there would be increased TSH and TRH.

I assume(d) that TSH/TRH would increase if thyroid hormone was being produced in a non-functional manner, or not being recognized - but since in this case it can't be produced at all due to lack of a subunit, the whole loop would shut down. Why would the body keep pushing TSH if there's no iodine to make thyroid hormone?

 

[docelh]

Body keeps producing TRH/TSH because there is no thyroid hormone in the blood to suppress it. That there is no iodide in the body is irrelevant to that particular mechanism because it's trained to react to thyroxine levels. When there is insufficient or no thyroxine in the blood, the hypothalamus/anterior pituitary just thinks... "time to tell the thyroid gland to produce some more."

In the case of goiter, the TSH binds to the thryoid gland cells; after this, it eventually becomes inactivated. Thus, the anterior pituitary has to produce more. TSH levels stay elevated.

 

[theWUbear]

Well stated - I feel better about that now. Thank you