The standard thyroid test ordered in primary care is TSH — thyroid-stimulating hormone. If TSH is within the reference range, the thyroid is declared healthy. The patient is sent home. This approach is so deeply embedded in medical practice that many physicians do not realize it leaves the majority of clinically meaningful thyroid information untested.
What TSH Actually Measures
TSH is produced by the pituitary gland in response to signals from the hypothalamus. When thyroid hormone levels in the blood fall, the pituitary increases TSH to stimulate the thyroid to produce more. When thyroid hormone levels rise, the pituitary reduces TSH. TSH, therefore, tells you about the pituitary's response to thyroid hormone levels in the blood — not about Free T3 activity in the tissues, not about conversion efficiency, and not about whether cells are actually using thyroid hormone appropriately.
A patient can have a perfectly normal TSH and simultaneously have: inadequate Free T3 at the tissue level, poor T4-to-T3 conversion driving symptoms, elevated Reverse T3 blocking T3 receptor sites, or positive thyroid antibodies indicating Hashimoto's thyroiditis that will eventually destroy the thyroid. TSH testing misses all of these.
The Complete Panel
Free T4 (fT4): The predominant thyroid hormone produced by the thyroid gland (approximately 80% of production). Free T4 is the unbound, biologically available fraction. Testing total T4 — which most basic panels do — includes bound T4 that cannot reach cells and provides a misleading picture.
Free T3 (fT3): The biologically active thyroid hormone — the form that actually binds to cellular receptors and drives metabolic function. T4 must be converted to T3 in the liver, gut, and peripheral tissues before it can be used. Many patients with adequate T4 have poor T4-to-T3 conversion, producing normal T4 and TSH alongside low Free T3 and persistent symptoms.
Reverse T3 (rT3): An inactive isomer of T3 produced by the liver when T4 conversion efficiency is compromised — often by chronic stress, inflammation, low iron, or low selenium. Elevated Reverse T3 competes with Free T3 at receptor sites, creating functional hypothyroidism even when Free T3 appears adequate. The Free T3:Reverse T3 ratio is a clinically useful marker of thyroid hormone bioavailability at the tissue level.
The Antibody Question
Thyroid peroxidase (TPO) antibodies and thyroglobulin antibodies are the diagnostic markers for Hashimoto's thyroiditis — the autoimmune destruction of thyroid tissue that is the most common cause of hypothyroidism in developed countries. Patients with positive antibodies may have completely normal TSH, T4, and T3 for years before thyroid destruction is sufficient to cause hypothyroidism. But their autoimmune activity is already damaging tissue and driving systemic inflammation. Early identification of Hashimoto's is critical for appropriate management.