TSH is a signal from the pituitary gland asking the thyroid to work. T3 and T4 are the hormones the thyroid actually produces. Because they form a feedback loop, doctors read them together.
A raised TSH with low T4 is commonly discussed as an underactive thyroid pattern, while a suppressed TSH with high T4 points the other way. Borderline results are common and often repeated after a few weeks.
Pregnancy, recent illness, biotin supplements and certain medicines can all influence readings, which is why a single result is rarely acted on alone.
Thyroid conditions are common and generally very manageable once identified. The important step is a consultation, not a conclusion drawn from a printed range.