More than just numbers on a blood test

Anyone who sees a doctor about thyroid problems will sooner or later receive blood results with abbreviations such as TSH, fT3 and fT4. These values are the most important tool for diagnosing and monitoring thyroid disease. Even so, many people remain unsure what exactly these numbers say and why they sometimes still have symptoms despite unremarkable results.

What TSH measures and why it doesn't tell the whole story

TSH stands for thyroid-stimulating hormone. It is not produced in the thyroid itself but in the pituitary gland. TSH controls how actively the thyroid works. If there is too little thyroid hormone in the blood, TSH rises to stimulate the thyroid. If there is enough, it falls again.

That sounds simple, but it has an important consequence: TSH is an indirect value. It does not measure how much thyroid hormone actually reaches the cells, only how the pituitary gland assesses the situation. This explains why some women continue to experience symptoms of an underactive thyroid despite a TSH within the normal range.

fT4 and fT3: the actual hormones

Thyroxine (T4) is the main hormone produced by the thyroid. In this form, however, it is still not very active. In the body, T4 is converted into triiodothyronine (T3), which is the truly active form. The small f in front of T4 and T3 stands for free and refers to the share of the hormones that is not bound to transport proteins and can actually enter the cells.

• fT4 shows how much hormone the thyroid produces
• fT3 shows how much active hormone is available in the body
• The conversion of T4 to T3 can be impaired by chronic stress, inflammation or nutrient deficiencies

Reference ranges and their limits

Laboratories give a reference range for each value, derived from measurements in a large population group. This means that anyone within the range is considered unremarkable. The problem is that this range is very wide and hardly takes individual differences into account. Some women already feel noticeably worse at a TSH of 3 mU/l than at 1 mU/l, even though both values are technically within the normal range.

The question is not only whether a value is in the green zone, but whether it is optimal for this particular person. That takes a conversation, not just a check of the numbers.

Antibodies as an important addition

If Hashimoto's is suspected, TPO antibodies and thyroglobulin antibodies should also be measured. Raised antibody levels point to an active immune reaction, even when TSH is still normal. They can help to detect Hashimoto's early, before thyroid function is measurably impaired.

Understanding your own lab results makes it easier to talk to doctors as an equal and to play an active part in your treatment. The values are pointers, not final verdicts.