Hyperthyroidism, or overactive thyroid gland, is another common thyroid condition. The most prevalent form is Graves’ disease in which the body’s autoimmune response causes the thyroid gland to produce too much T3 and T4. Symptoms of hyperthyroidism can include weight loss, high blood pressure, diarrhea, and a rapid heartbeat. Graves’ disease also disproportionately affects women and typically presents before the age of 40.4

The normal values for the serum T4:T3 ratio are seldom discussed in the literature because measurement of serum T3 levels is not a recommended outcome in hypothyroidism (1). In a large study of approximately 3800 healthy individuals (4), the serum free T4:free T3 ratio was around 3, as opposed to a ratio of 4 in more than 1800 patients who had undergone thyroidectomy and were receiving l-thyroxine monotherapy. The corresponding serum free T4:free T3 ratio in patients continuing to receive desiccated thyroid is not well-defined, but the serum total T4:T3 ratio is known to be low (28, 50). In one study, the serum total T4:total T3 was about 40 in patients receiving desiccated thyroid and about 100 in those taking l-thyroxine monotherapy (60). Of course, this is affected by the timing of blood collection in relation to the timing of l-triiodothyronine administration, which is not commonly reported. Other key factors are the well-known poor reproducibility of the serum total T3 assay (61) and the interferences with direct measurement of free T3 (5).


For starters, consider the effect that hypothyroidism can have on weight. Hypothyroidism (also called low thyroid or underactive thyroid) is marked by insufficient hormone production in the thyroid — the butterfly-shaped gland located at the bottom-front of your neck. This gland affects the body’s metabolic processes, and often, sudden weight gain is an early sign of low thyroid.

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Please Note: The material on this site is provided for informational purposes only and is not medical advice. Always consult your physician before beginning any diet or exercise program.

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