What do you need to know about the crucial thyroid hormone, thyroxine (T4)?

Thyroxine (T4), also known as tetraiodothyronine, is the thyroid gland’s second most important hormone after triiodothyronine (T3). Thyroxine accounts for 80% of this gland’s hormonal output. It is a vital compound that is involved in numerous bodily processes. Every organ system is affected by thyroxine, including the heart, brain, bones, digestive system, and metabolism.


  • regulation of metabolism,
  • carbohydrate metabolism,
  • protein anabolism,
  • breakdown of fats,
  • normal bone growth,
  • heart work,
  • lung work,
  • brain development,
  • mood,
  • fertility, menstrual cycle, ovulation and potency.

The majority of thyroxine is converted to its active form (triiodothyronine) by organs other than the thyroid gland, such as the liver and kidneys. Thyroxine is secreted by the thyroid gland, the anterior pituitary gland, and the hypothalamus working together. TSH, a thyroid-stimulating hormone produced in the anterior pituitary gland, is the primary regulator of thyroxine release in the thyroid gland. When thyroxine levels rise, the concentration of TSH in the blood decreases. When the thyroid gland does not produce enough hormones, the TSH level rises in order to stimulate the thyroid gland to function properly.

Thyroxine (T4) test – norms

To determine whether your thyroid is working properly, you should have your thyroxine (T4) and TSH levels checked, as well as triiodothyronine (T3) levels. However, determining free thyroxine (fT4) is advised.

The normal concentration of free thyroxine (fT4) is:

  • in an adult: 9.5-20 pg/ml (12-24 pmol/l),
  • in a child:
    up to 2 years old: 8-20 pmol/l,
    2-10 years old: 10-26 pmol/l.

Based on the results, it is possible to not only determine thyroid function (whether hypothyroidism or hyperthyroidism exists), but also to identify the possible cause of abnormalities and track the effects of treatment.

When should I test my thyroxine level?

If thyroid disease is suspected, thyroxine testing should be done. The presence of goitre, exophthalmos, or thyroid nodules is an indication. When symptoms of hypofunction or hyperfunction of this gland appear, thyroxine should be tested as well:

  • unplanned weight gain or loss,
  • excessive sweating or feeling cold.
  • skin problems,
  • difficulties with concentration and memory,
  • constant tiredness or overexcitation,
  • problems with getting pregnant or potency,
  • irregular menstrual cycles,
  • tachycardia or bradycardia,
  • low mood (depression) or irritability and nervousness.

The basic idea is that hyperthyroidism accelerates various body processes, resulting in faster metabolism, a faster heart rate, a feeling of being hot, and sweating. However, despite a healthy lifestyle, a patient with hypothyroidism may experience bradycardia, intolerance to lower temperatures, or difficulty maintaining a normal body weight.

High thyroxine levels – causes and symptoms

Hyperthyroidism is diagnosed when there is an excess of thyroxine in the body. Graves’ disease is the most common cause of high thyroxine, while thyroiditis, thyroid nodules, a benign tumour, certain medications, and pituitary adenoma are all possible causes. Thyrotoxicosis is defined as an excess of thyroxine in the body, with potential side effects such as goitre, menstrual disorders, diarrhoea, and tachycardia.

Low thyroxine levels – causes and symptoms

Low thyroxine levels mean hypothyroidism, with TSH usually elevated and fT4 below normal. The cause of thyroxine deficiency can be autoimmune disease, postpartum thyroiditis, atrophic thyroiditis, viral thyroiditis, thyroid malformations in children, pregnancy, use of certain medications, or abnormalities in the pituitary or hypothalamus. People with thyroxine deficiency may experience intolerance to lower temperatures, slow heart rate, unjustified weight gain, depression, difficulties in getting pregnant, or problems with potency.


Author: DoctorMaryam.org

4th Professional Medical Student. Karachi Medical and Dental College.

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