Synthesis

Iodine is needed for the synthesis of thyroid hormone. Our main source of iodine is from salt which has been enriched with iodine and which is an ingredient in our bread and other foods. As it happens, our diets generally contain too little iodine. This addition of iodine helps prevent iodine deficiency. Iodine can also be found, in small quantities, in tap water and in sea fish, shellfish and crustaceans, and in sea salt. The red food coloring E127, which is used in sauces, candy and preserved fruits, also contains iodine.

The thyroid gland synthesizes mainly T4 hormone and a small amount of T3. The amino acid tyrosine and iodine are needed for the synthesis of T4 and T3. Iodide, the form of iodine present in the blood, is transported from the blood via the thyroid gland cell, or follicular cell, into the thyroid follicle (the cavity), where it is converted into the iodine atom.
The follicular cell also makes thyroglobulin and transports it to the thyroid follicle. Thyroglobulin contains tyrosines. With the help of the enzyme TPO, each tyrosine molecule can bind to two iodine atoms. If tyrosine binds with one iodine atom, then monoiodotyrosine or MIT is formed. If tyrosine binds with two iodine atoms, then diiodotyrosine or DIT is produced.

T4 has four iodine atoms and is produced when two DITs (2+2 iodine atoms) bind together. T3 has three iodine atoms and is produced when one DIT and one MIT (2+1 iodine atoms) bind together. Although the thyroid gland produces much less T3 than it does T4, T3 is, in fact, the active form of the hormone. T4 is the inactive form of thyroid hormone and becomes active only when it is converted to triiodothyronine (T3).

This conversion occurs at locations outside of the thyroid gland, usually in the liver and kidneys but also in tissues or organs where the hormone is needed. During the conversion of T4 into T3, the enzyme deiodinase splits off one iodine atom. The amount of T3 that is produced as a result of this conversion is approximately 80% of the total amount of T3 in your body. The remaining 20% is produced in the thyroid gland itself.
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Secretion

The secretion of thyroid hormone into your blood is a process that is very precisely regulated so that you always have the perfect amount of hormone in your body. Not too much and not too little. The secretion of thyroid hormone is regulated by the pituitary and by the hypothalamus, two small glands in your brain which also synthesize hormones. The hypothalamus synthesizes TRH (thyrotropin-releasing hormone). TRH signals the pituitary to make TSH (thyroid-stimulating hormone). TSH, in turn, makes sure that the thyroid gland secretes thyroid hormone into the bloodstream and that more hormone is then synthesized to replace it.

The amount of thyroid hormone in your blood and tissues is continuously being 'measured' by the hypothalamus. Once the thyroid hormone level is sufficient, the production of TRH declines. As a result of this, the pituitary produces less TSH which means that the thyroid gland ultimately synthesizes and secretes less thyroid hormone. After all, there is already enough of it to go around. The control of the thyroid, including this feedback loop, is called the hypothalamus-pituitary-thyroid axis.

Transport through the bloodstream

In the blood, the majority of the thyroid hormones T4 and T3 are bound to proteins, such as the transport protein TBG (thyroxine-binding globulin). A very small portion of the T4 is not bound to a transport protein and is called 'free T4' (FT4).

What are the effects of thyroid hormone?

Thyroid hormone affects virtually all the tissues and many of the processes in your body, such as your metabolism and energy consumption, longitudinal growth, the development of the brain during the first three years of life and, after that, brain function. The hormone works as a kind of gas pedal and ensures that your 'engine' runs faster or slower. In the table below, you can see which organs and processes are affected by the thyroid hormone and what the consequences are if your thyroid gland works too slowly (hypo) or too quickly (hyper).

Effect on the nervous system	Hypo	Hyper
- Stimulates the growth and development of the brain (during pregnancy and the first years of life) - Stimulates alertness, responsiveness and activity levels - Boosts memory and learning capacity	Tired, sluggish, desire to sleep Difficulties concentrating and focusing Feeling gloomy Headache Visual problems Hearing loss	Being nervous, stressed and agitated Feeling irritable and unstable Difficulty concentrating and focusing
Effect on the skeletal system
- Stimulates the growth of bone (long bones)	Small for age; retarded growth	Tall for age; accelerated growth
Effect on the heart	Hypo	Hyper
- Increases heart rate - Increases the amount of blood that the heart pumps per minute	Slow heart rate	Increased heart rate, heart palpitations, high blood pressure
Effect on respiration
- Increases the rate of respiration		Rapid breathing
Effect on the metabolism
- Increases the fat metabolism (resulting in reduced cholesterol levels) - Increases the amino acid metabolism - Increases the vitamin metabolism - Increases the speed of the basal metabolism (also results in increase in heat and sweat production)	Overweight Constipation Feeling cold Late onset of puberty Menstrual disorders	Underweight Often feeling hungry Diarrhea Feeling hot, sweaty, clammy
Other effects	Hypo	Hyper
- Increases sensitivity to catecholamines		Heart rate, blood pressure and blood sugar levels increase Trembling hands
Other complaints	Dry, pale skin Hair loss Secretion of milk (lactation w/o pregnancy) Hearing loss Joint pain	Red cheeks (flushes) Enlarged thyroid gland Bulging eyes Muscle weakness