Thyroid Function Blood Test Kit

Thyroid stimulating hormone (TSH) is produced by the pituitary gland and acts as the master controller of thyroid function. TSH tells the thyroid gland how much hormone to produce. The pituitary constantly monitors thyroid hormone levels in the blood and adjusts TSH output accordingly: when thyroid hormones are low, TSH rises to stimulate the thyroid to produce more; when thyroid hormones are high, TSH falls to slow production. This negative feedback loop makes TSH the most sensitive early indicator of thyroid dysfunction—often the first marker to become abnormal, sometimes before the thyroid hormones themselves move outside normal range. An elevated TSH indicates that the thyroid is underperforming (hypothyroidism). The pituitary is working harder to stimulate a sluggish thyroid. Hypothyroidism is common, affecting approximately 2% of the UK population, with higher rates in women and with increasing age. Symptoms include fatigue, weight gain, feeling cold, dry skin, constipation, hair loss, depression, and difficulty concentrating. A low or suppressed TSH indicates that the thyroid is overactive (hyperthyroidism). The pituitary is backing off because thyroid hormone levels are already too high. Symptoms include weight loss, anxiety, tremor, palpitations, heat intolerance, diarrhoea, and difficulty sleeping. Normal TSH is typically 0.4-4.0 mU/L, though some guidelines suggest 0.4-2.5 mU/L may be more appropriate. TSH between 4-10 mU/L with normal Free T4 is termed "subclinical hypothyroidism"—this may or may not require treatment depending on symptoms, antibody status, and other factors. TSH below 0.1 mU/L with elevated Free T4/T3 indicates overt hyperthyroidism. Results outside the normal range may need a follow-up with your GP.

Free T4 measures the unbound, biologically active fraction of thyroxine circulating in your blood. Thyroxine (T4) is the main hormone produced by the thyroid gland, accounting for about 80% of its hormonal output. Most T4 in blood is bound to carrier proteins (primarily thyroxine-binding globulin, or TBG) and is inactive; only the free fraction (approximately 0.03%) is available to enter cells. This "free" T4 is what we measure because it reflects the hormone actually available to your tissues, unaffected by variations in binding protein levels. T4 is sometimes called a "storage" or "prohormone" because it must be converted to the more active T3 (triiodothyronine) to exert its full effects on cells. This conversion happens in peripheral tissues (liver, kidney, muscles, brain) by enzymes called deiodinases. Free T4 provides a good overall measure of thyroid hormone production. Combined with TSH, it allows classification of thyroid status: high TSH with low Free T4 confirms primary hypothyroidism; low TSH with high Free T4 confirms hyperthyroidism; high TSH with normal Free T4 indicates subclinical hypothyroidism. Normal Free T4 is typically 9-25 pmol/L, though optimal may be in the upper half of this range for many people. When interpreting Free T4, always consider it alongside TSH—the combination tells a clearer story than either alone. Some people feel well with Free T4 at the lower end of normal; others need it in the upper range. Individual variation exists, and clinical symptoms matter as much as laboratory numbers. Results outside the normal range may need a follow-up with your GP.

Free T3 measures the unbound, active fraction of triiodothyronine. T3 is the most metabolically active thyroid hormone—approximately 3-4 times more potent than T4 at the cellular level. T3 is the hormone that actually enters cell nuclei and switches on thyroid-responsive genes, affecting metabolism, energy production, protein synthesis, and countless other cellular functions. Only about 20% of T3 is produced directly by the thyroid gland; the majority (approximately 80%) is produced by conversion of T4 to T3 in peripheral tissues. Including Free T3 in a thyroid panel provides additional information beyond TSH and Free T4 alone. It's particularly useful for detecting hyperthyroidism (where Free T3 may be elevated earlier or more prominently than Free T4, especially in "T3 toxicosis"), assessing T4-to-T3 conversion efficiency (some people convert poorly, leading to low T3 despite adequate T4), and evaluating symptoms in patients on thyroid replacement therapy who feel unwell despite "normal" TSH and Free T4. Some patients feel their T3 levels are important for their wellbeing, though this remains somewhat debated in mainstream endocrinology. Normal Free T3 is typically 3.5-6.5 pmol/L. Low Free T3 with normal TSH and Free T4 often indicates non-thyroidal illness rather than a primary thyroid problem—the body downregulates T3 production during illness, starvation, or severe stress as a protective mechanism. Elevated Free T3 with suppressed TSH is a key finding in hyperthyroidism. In hypothyroid patients on levothyroxine, Free T3 usually remains normal even if Free T4 is at the higher end, due to peripheral regulation of conversion. Results outside the normal range may need a follow-up with your GP.