The standard thyroid function test measures thyroid-stimulating hormone, TSH, produced by the pituitary gland. A normal TSH is taken to indicate that the thyroid gland is producing adequate hormone and the body is satisfied with the result. The problem is that this interpretation contains several assumptions that do not hold in every case, particularly in adults over 35 with other hormonal disruptions, chronic stress, or autoimmune disease.
The Five Markers That Provide a Complete Picture
- TSH: the pituitary signal; indicates whether the brain perceives sufficient thyroid hormone, but not whether the tissue response is adequate
- Free T4: the primary thyroid secretion; elevated if the thyroid is overproducing, low if it is underproducing
- Free T3: the active hormone that enters cells; can be low even when TSH and T4 are normal if conversion is impaired
- Reverse T3: an inactive form of T3 that can accumulate during chronic stress and illness and competitively block T3 receptors; standard panels never include this
- Thyroid peroxidase antibodies (TPO-Ab): positive in Hashimoto's thyroiditis, the most common cause of hypothyroidism in developed countries; standard panels frequently omit this
The Reference Range Problem
The normal range for TSH in most UK laboratories is approximately 0.4 to 4.5 mIU/L. This range was derived from population studies that included people with undiagnosed thyroid dysfunction in the healthy reference group. Functional medicine practitioners and some endocrinologists argue that optimal TSH in adults seeking to feel well and perform well is more likely to sit between 1.0 and 2.5 mIU/L. A TSH of 4.0 is technically within the reference range and technically normal, but may represent significantly suboptimal thyroid function for that individual.
Sources
- Garber, J.R., et al. (2012). Clinical practice guidelines for hypothyroidism in adults. Thyroid, 22(12), 1200-1235.
- Wartofsky, L., & Dickey, R.A. (2005). The evidence for a narrower thyrotropin reference range is compelling. Journal of Clinical Endocrinology and Metabolism, 90(9), 5483-5488.
- Vanderpump, M.P.J., et al. (1995). The incidence of thyroid disorders in the community: a twenty-year follow-up of the Whickham Survey. Clinical Endocrinology, 43(1), 55-68.