What is TSH?
Thyroid-stimulating hormone (TSH) is produced by the pituitary gland, a small structure at the base of the brain. Its job is to signal the thyroid gland to produce thyroid hormones — primarily thyroxine (T4) and triiodothyronine (T3). When thyroid hormone levels in the blood are low, the pituitary releases more TSH. When they are high, it releases less.
This feedback loop makes TSH a useful indirect measure of thyroid function — but only an indirect one.
Why TSH alone is often insufficient
The most common thyroid complaint doctors hear in India is some version of: "My TSH was normal, but I still feel exhausted." This frustration is understandable and clinically meaningful.
TSH reflects the pituitary's reading of thyroid hormone levels, not the tissues' access to those hormones. Several things can make TSH appear "normal" while a patient is genuinely symptomatic:
Free hormone levels may be suboptimal. TSH reference ranges were established in large population studies that may include participants with subclinical thyroid dysfunction. A patient whose free T4 sits at the lower end of the reference range may experience symptoms even with a TSH of 3.0 mIU/L.
Conversion problems. T4 is largely inactive; it must be converted to the more active T3 in peripheral tissues. Some patients, particularly those with certain genetic variants of the deiodinase enzyme, convert T4 to T3 less efficiently. TSH and T4 may look normal while T3 is low.
Hashimoto's thyroiditis. This autoimmune condition causes fluctuating thyroid function as the immune system attacks the gland. TSH levels can swing from low to high over weeks. A single TSH measurement at a moment when function is relatively preserved may miss an active inflammatory process.
What a complete thyroid evaluation includes
A doctor evaluating thyroid function comprehensively typically considers:
- TSH — the initial screening tool
- Free T4 (FT4) — the primary thyroid hormone produced by the gland
- Free T3 (FT3) — the active form of thyroid hormone; particularly useful when symptoms persist despite normal TSH and T4
- Anti-TPO antibodies — elevated in most patients with Hashimoto's thyroiditis; indicates an autoimmune process even if TSH is currently normal
- Anti-thyroglobulin antibodies — a second autoimmune marker, less specific than anti-TPO but useful in certain presentations
The decision of which tests to order depends on the clinical picture. A patient with a strong family history of autoimmune disease, symptoms inconsistent with the TSH number, or a previous diagnosis of Hashimoto's warrants a broader panel than someone presenting with a mildly elevated TSH and no other features.
Understanding your TSH number
Reference ranges differ between laboratories in India. A commonly used range is 0.4–4.0 mIU/L, but some laboratories use ranges as wide as 0.3–5.5 mIU/L. Two patients with TSH values of 4.1 and 5.5 may receive different classifications depending on which laboratory reference range applies.
Several factors can transiently affect TSH:
- Time of day: TSH is highest in the early morning and lowest in the late afternoon. A TSH measured at 4 PM will be lower than one measured at 7 AM from the same patient on the same day.
- Recent illness: acute illness suppresses TSH temporarily, a phenomenon called non-thyroidal illness syndrome.
- Certain medications: biotin supplements (common in India for hair loss) can cause falsely elevated or suppressed TSH in some assay platforms.
- Pregnancy: TSH reference ranges shift substantially during pregnancy and must not be interpreted with standard adult ranges.
Subclinical hypothyroidism: the grey zone
A TSH between 4.5 and 10.0 mIU/L with normal free T4 is classified as subclinical hypothyroidism — a term that can be misleading, because many patients in this range are symptomatic. The decision to treat with levothyroxine in this range is individualised. Your doctor considers:
- Symptoms and their impact on daily functioning
- Presence of anti-TPO antibodies (positive antibodies increase the risk of progression)
- Age and cardiovascular risk factors
- Fertility plans
There is no universal answer. The right decision for one patient may not be right for another with the same TSH number.
What to ask your doctor
If you have received a thyroid test result and are uncertain what it means, these questions are worth discussing:
- What is my TSH, and where does it sit within the laboratory's reference range?
- Have free T4 and free T3 been measured? If not, would they be informative given my symptoms?
- Have anti-TPO antibodies been checked? If positive, what does that mean for my management?
- If I am on levothyroxine, is this dose achieving the intended response — and how is that being assessed beyond TSH alone?
A doctor who takes time to answer these questions with specificity, and who grounds the answer in your individual clinical picture rather than a population reference range, is practising the kind of thyroid care that most patients have not yet received.