Natural thyroid production will be completely shutdown for a good period of time after using T3, but it will eventually recover.
I still like short cycles b/c you shouldnt have to be on T3 a long time to lose weight--it is a good finisher at the end of a cutter. Short cycles do less damage to the organ's ability to regulate T3 especially after so many cycles. Long cycles at high doses will shut your thyroid down for longer periods before coming back. However, your Thyroid will always continue to work it may work as well with greater rebound effect. With cycles of short length at 25mcgs you are giving your thyroid a push to boost your metabolism and especially protein synthesis. It is easier to recover from these quicker, take time off and start again. They are especially good for 16 weeks AAS cycles. Also, tapering down helps with rebound issues.
N Engl J Med. 1975 Oct 2;293(14):681-4. Related Articles, Links
Recovery of pituitary thyrotropic function after withdrawal of prolonged thyroid-suppression therapy.
Vagenakis AG, Braverman LE, Azizi F, Portinay GI, Ingbar SH.
The pattern of thyrotropin secretion was analyzed in seven euthyroid women, before and after withdrawal of long-term thyroid hormone, by serial measurements of thyroid 131l uptake, serum thyroxine, tri-iodothyronine, and thyrotropin concentrations, and the response to thyrotropin-releasing hormone. During exogenous hormone administration, 131l uptake was suppressed, and serum thyrotropin concentrations before and after administration of thyrotropin-releasing hormone were undetectable. After withdrawal of exogenous hormone, thyrotropin secretory function was transiently impaired, as indicated by undetectable basal thyrotropin concentrations together with absence of response to thyrotropin-releasing hormone, and subsequently by normal values of basal thyrotropin concentration and normal responses to releasing hormone while serum thyroxine and tri-iodothyronine concentrations were subnormal. Decreased thyrotropin reserve persisted for two to five weeks. Detectable values of serum thyrotropin (less than 1.2 muU per milliliter) and a normal 131l uptake usually occurred concurrently in two to three weeks. Serum thyroxine concentration returned to normal at least four weeks after hormone withdrawal.
PMID: 808728 [PubMed - indexed for MEDLINE]