Apparently some people can remain on T3-only indefinitely and actually feel fine, or certainly better than they ever did on T4 medications. Others, however, become worse, because the high T3 levels have side effects and create imbalances in other hormones. In men, high T3 levels will cause sex hormone binding globulin (SHBG) to rise, the production rate of estradiol to rise, and the metabolic clearance rate of estradiol to fall. [1, 52] The net effect is higher estradiol levels, which can have a disastrous effect on men’s hormone levels. In men, a hyperthyroid state can elevate estradiol and result in gynecomastia (male breasts).  Premature ejaculation seems to be a problem for hyperthyroid men, and delayed ejaculation a problem for hypothyroid men.  High SHBG in men also correlates with osteoporosis and a higher fracture risk. 
SHBG also rises in women taking additional T3. The metabolic clearance rate of estradiol decreases  and anecdotally, women have reported painful breasts on the T3-only protocol. Women’s testosterone levels also decrease on this protocol. This may be desirable if a woman exhibits high androgen symptoms like facial hair, infertility, etc., but undesirable if levels become too low. 
In women, estrogen and progesterone can also raise SHBG if taken orally , so if a woman is on these other hormones and following the T-3 only protocol, her system can become quite imbalanced. Hormonal imbalance is often the cause of both physical and emotional symptoms.
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