T4 (thyroxine) is the primary hormone secreted by the thyroid. About 80% of total output.
Mechanism: Relatively inactive on its own, T4 acts mostly as a circulating reservoir. It's deiodinated to T3 at the tissue level by deiodinase enzymes. 'Free' T4 measures the unbound, biologically available fraction.
Lab range: 0.8–1.8 ng/dL standard; many target mid-to-upper range.
Caveats: Synthetic T4 (levothyroxine, Synthroid) is the standard hypothyroid medication, but it doesn't address conversion problems. T4-only therapy fails roughly 10–15% of patients due to poor peripheral conversion.