They ordered a free t4 and a tsh-3rd generation. when do i need a free t3 (liothyronine)?

Probably after FT4.: Hyperthyroidism is usually monitored by TSH and free t4. If these check out normal, then a T3 (liothyronine) or free T3 (liothyronine) might be measured. The free T4 is the amount of active hormone available to the body. T3 (liothyronine) is actually more potent than T4 but its half life is low so very little is there. Also, TSH would help tell if there was trouble with your thyroid. If not, then no more readings may be needed.

Immediately: Hypothyroidism can be primary - caused by a weak thyroid gland or secondary to other hormonal problems, like a pituitary problem. Or your thyroid can become dysfunctional secondary to high insulin, estrogen or cortisol. If you have functional hypothyroidism, this problem can go undetected by routine screening tests. More extensive testing can often reveal abonormalities. This is very common.