TSH levels affected by T4/T3

[gimel]

Is the Pituitary output of TSH dependent on T4 levels or T3 levels, or both?  Thanks.

[stacey555]

It souns like what I have I think.  My tsh is low which would indicate hyperthyroid but my t4 an t3 are low so I think that indicates pituitary.  Let me know what happens

[gimel]

Just a followup to previous question.   Based on the high degree of correlation for TSH on FT4, ( correlation coefficient of .9)  it appears that FT4 is the primary factor.  So, is it possible that a hypo t patient that is being treated with synthroid could have  FT4 level above the range, with a resultant very low TSH, and yet have a low FT3, if there is a problem converting T4 to T3?  This might account for my situation, because I have been told that my TSH indicates hyper t, yet I have no such symptoms, and my T3 is on the low end of the range.  Thanks for your interest and help.



Figure 6-7. Correlation of the serum TSH concentration and the free thyroxine index (FT4I) in three individuals given increasing doses of L-T4. Note the logarithmic correlation between TSH and FT4I and the variable individual requirement of free T4 to normalize the TSH level. Normal ranges are included in the heavy lined box and those for subjects on L-T4 replacement in the light liquid box. (From D. Sarne and S. Refetoff, Endocrinology, L.J. DeGroot (ed). 1995, Grune & Straton Inc.)

[Mark Lupo, M.D.]

both