Thyroid blood tests: why TSH often comes first

Recent Canadian questions keep returning to the same point: why does a requisition sometimes include only TSH instead of a “full panel” with T4, T3 and antibodies? The answer is not that the other tests are being ignored. In many situations, TSH is the most useful starting point; what comes next depends on symptoms, history, medications, pregnancy and the first result.

TSH often comes first because it guides the initial thyroid assessment. Free T4, T3 or antibody tests may be added when the clinical context and results support them. (Read more)

What TSH can show — without diagnosing on its own

TSH is a hormone made by the pituitary gland that helps regulate thyroid activity. When primary thyroid disease is suspected, measuring it often provides the first useful signal. The result still has to be read with the laboratory range, symptoms, health history and other relevant factors. A single value, whether in or out of range, should not be interpreted in isolation or used for self-diagnosis.

Why free T4 and T3 are not always on the first requisition

Choosing Wisely Canada explains that free thyroid hormone tests are useful in specific situations, but not automatically in every case. Some laboratories use a reflex approach: free T4 may be processed when TSH is outside the expected range or when clinical justification is provided. Free T3 answers more targeted questions. More boxes checked do not necessarily produce a better answer; the right set of tests depends on the clinical question.

Routine screening is different from investigating symptoms

The Canadian Task Force on Preventive Health Care recommends against routine thyroid screening for nonpregnant adults who have no symptoms or risk factors. That recommendation does not apply in the same way when someone has symptoms, a known thyroid condition, certain medications, relevant surgery or radiation history, or pregnancy. In those settings, the clinician selects testing for the individual context instead of applying one standard panel to everyone.

Prepare the conversation, not a test list to impose

Before the visit, gather what can genuinely change the decision: your symptoms and their timeline, prior results with dates, personal and family history, and a complete list of medications and supplements. Also mention pregnancy, plans for pregnancy, known thyroid disease or recent treatment changes. Do not change a medication or supplement to influence a test unless the professional managing your care has told you to do so.

What an online request can — and cannot — do

MedVibe lets you request blood work for review by a licensed clinician, who considers the reason, history and relevant tests. A requisition may be issued when clinically appropriate, but neither a specific test nor a requisition is promised. Some symptoms, pregnancy-related questions, complex results or needs for a physical examination may lead to in-person care or another follow-up path.

This guide is for adults and provides general information; it does not select your tests or interpret an individual result. Children, pregnancy, known thyroid disease, thyroid surgery, some pituitary conditions and certain medications require a tailored assessment. For non-urgent health advice in Ontario or Quebec, call 811. For a severe, sudden or potentially life-threatening symptom, call 911 or go to an emergency department.