Why TSH alone is insufficient
TSH (thyroid stimulating hormone) is produced by the pituitary gland as a signal to the thyroid. When thyroid function declines, TSH rises to stimulate more production — it is an upstream signal, not a direct measure of thyroid hormone levels. TSH can remain "normal" while Free T3 (the active hormone) is low due to impaired T4-to-T3 conversion. Always order the full panel.
What each test measures
- TSH (Thyroid Stimulating Hormone) — pituitary signal; rises when thyroid output is low, falls when it's high; sensitive for detecting over/under-function but doesn't tell you about conversion
- Free T4 (Free Thyroxine) — the main secretory product of the thyroid; the storage/prohormone form; needs to be converted to T3 to be active; relatively stable in circulation
- Free T3 (Free Triiodothyronine) — the biologically active form; enters cells and binds thyroid hormone receptors; drives metabolism, heart rate, body temperature, cognition; the hormone that actually does the work
The T4→T3 conversion — the selenium connection
Free T4 is converted to the active Free T3 in peripheral tissues (liver, kidney, muscle, brain) by selenium-dependent deiodinase enzymes (DIO1, DIO2, DIO3). Selenium deficiency — common in India due to selenium-poor soils — impairs this conversion, creating a pattern of:
- Normal TSH (pituitary sees adequate T4)
- Normal Free T4 (thyroid producing enough)
- Low-normal or low Free T3 (conversion impaired)
- Symptoms: fatigue, cold intolerance, weight gain, brain fog
This "low T3 syndrome" or "euthyroid sick syndrome" is only detectable when Free T3 is measured. Supplementing selenium 100–200 mcg/day (as selenomethionine) can meaningfully improve T4→T3 conversion.
Thyroid panel interpretation table
| Marker | Lab Reference (India) | Longevity Optimal | Action if Outside Optimal |
|---|---|---|---|
| TSH | 0.5–4.5 mIU/L | 1.0–2.5 mIU/L | TSH above 2.5 with symptoms: investigate Free T3, T4, TPO antibodies |
| Free T4 | 0.8–1.8 ng/dL | Mid-upper range (1.1–1.6 ng/dL) | Low Free T4 with high TSH: hypothyroidism; physician evaluation |
| Free T3 | 2.0–4.4 pg/mL | Upper half of range (3.2–4.4 pg/mL) | Low Free T3 with normal T4/TSH: selenium deficiency; optimise conversion |
| TPO Antibodies | <35 IU/mL (negative) | <35 IU/mL | Elevated: Hashimoto's thyroiditis — iodine caution; selenium beneficial; physician management |
Subclinical hypothyroidism in India
Subclinical hypothyroidism — TSH mildly elevated (4.5–10 mIU/L) with normal Free T3 and T4 — affects 10–15% of Indian adults, with women disproportionately affected (3–4:1 female:male ratio). Symptoms can still occur even with normal T3/T4 levels: fatigue, weight gain, cold intolerance, constipation, cognitive slowing, and dry skin.
Management is individualised — not all subclinical hypothyroidism requires medication. Selenium, iodine optimisation, and addressing Hashimoto's antibody load are first-line before pharmacotherapy in mild cases.
Hashimoto's thyroiditis — the India context
Hashimoto's autoimmune thyroiditis is the most common cause of hypothyroidism in India. The immune system attacks thyroid tissue, progressively reducing thyroid function. Diagnosis requires TPO antibodies (anti-thyroid peroxidase) — often markedly elevated (hundreds to thousands IU/mL).
High-dose iodine supplementation can worsen Hashimoto's autoimmunity by increasing the immunogenicity of thyroglobulin. Keep iodine at the RDA of 150 mcg/day from food and supplements combined — do not megadose. Selenium 100–200 mcg/day is the preferred thyroid-supportive supplement in Hashimoto's, and RCTs show it reduces TPO antibody levels over 3–6 months.
How often to test
Annually as part of a comprehensive health panel. If symptoms of thyroid dysfunction are present (fatigue, hair loss, weight changes, cold intolerance, cognitive slowing): test at symptom onset. If on thyroid medication: retest every 6–8 weeks when adjusting dose, then every 6 months when stable.
Frequently asked questions
Is TSH alone enough to test thyroid health in India?
No. TSH is a pituitary signal, not a direct measure of thyroid hormones. Always test TSH + Free T3 + Free T4 together. If Hashimoto's is suspected, add TPO antibodies. TSH alone misses low T3 syndrome and subclinical cases.
What is subclinical hypothyroidism?
Mildly elevated TSH (4.5–10 mIU/L) with normal Free T3 and T4. The thyroid struggles to maintain output so the pituitary overdrives it. Symptoms often still occur. Affects 10–15% of Indian adults. Not always requiring medication — address selenium, selenium, and antibodies first.
Why do I need Free T3 and Free T4 tests?
Free T4 shows thyroid output; Free T3 shows what actually reaches your cells. Selenium-dependent conversion from T4 to T3 can be impaired, giving low T3 with normal T4 and TSH — only visible when all three are measured.
Does selenium help thyroid T3 conversion in India?
Yes — deiodinase enzymes converting T4 to T3 are selenoproteins. Selenium deficiency (common in India) impairs this conversion. Supplementing selenomethionine 100–200 mcg/day improves T3 levels and reduces TPO antibodies in Hashimoto's patients. Do not megadose — toxicity occurs above 400 mcg/day.