The answer to this seemingly innocuous question is not that simple at all. We know ,sinus tachycardia is a common expression of thyroid excess while sinus bradycardia is the hall-mark of thyroid depletion. So obviously , the first thing that would strike us is , there must be something cooking between thyroxine and the SA node .
We realised much later , there is no direct action of thyroid hormone on the SA node instead it has a crucial interaction with adrenergic system which has the major influence on chronotropy.
How does thyroid interact with adrenergic system ?
Thyroxine (T4) is an inactive molecule , it has to get converted to T3 for its action. This conversion takes place inside target cells like myocytes, pacemaker cells (of course it has action on virtually any metabolically active cell ! )
The most important point to remember is , unlike catecholamines the thyroid receptors are located in the surface of the nucleus inside the cell , instead of cell membrane . Surprisingly T4 does not require any specialised transporter to enter the cell.It simply diffuses into the bi-lipid layer of cell membrane as T4 is immensely lipid soluble. . After entering the cell in the cytoplasm it gets converted into T 3.
This T 3 is attracted towards the nucleus. Once it is attached to nucleus , it brings about changes in the gene configuration and through messenger RNA results in new protein generation . These cellular elements are vital for maintaining the ionic channels and ports and anti-ports.Among these the most important is adrenergic receptor molecule , an its signal system namely the GTP/Adenyl cyclase/Cyclic AMP units in the cell membrane .
Thyroxine is a physiological hormone required to maintain these adrenergic receptor complex on day-to-day basis. (It can be called as cell servicing hormone )
The circulating catecholamine’s action is heavily influenced by the thyroid hormone status. Sympathetic nervous system is the live wire for human biological system. So , when thyroid is in excess the entire metabolism of cell is increased and vice versa happens.With the close interaction with adrenergic system , one can understand how thyroid excess causes anxiety state and depletion causes depression.
Coming back to heart ,
- Thyroid hormone up-regulates beta receptors in cell membrane and augments the action of epinephrine and result sinus tachycardia .It can have positive inotropic action as well (Hyperdynamic state )
- Aguments intracardiac conduction.
- The action of thyroid hormone on heart can well extend to the pathological phase, where in it can cause multiple ventricular ectopics and atrial fibrillation. (Note the striking similarities between these arrhythmias and adrenergic arrhythmias !)
What is time frame for thyroid hormone action ?
Obviously thyroid hormone can not have a rapid onset action since it invites the nuclear synthesis (Like steroid hormones) it may take few weeks time for thyroid hormone to express its effects.
How does beta blockers exert its action in thyrotoxicosis ?
This occurs in two ways.
- The beta blockers occupy the adversely up-regulated dense beta receptors of cell membrane and prevents excess adrenergic action.
- There is some evidence beta blockers prevent conversion of T4 into T3 .This seems to be less important than its direct sympathetic blockade.
For effective control of thyrotoxicosis one need to administer beta blocker in combination with anti-thyroid drugs.
Will calcium blockers be effective in controlling the tachycardia of thyrotoxicosis ?
No it is not . It may reduce the heart rate a little but never to the extent of beta blocker. This is another indirect evidence for the interaction between thyroxine and adrenergic system.
If thyroid hormone is able to potentate the circulating catecholamine action why not it be used as a
positive Inotropic in cardiac failure ?
A very valid question. It was tried by many researchers especially in dilated cardiomyopathy. .For some reason it has not worked well , except in patients with associated with hypothyroid state.I personally believe thyroid hormone must have a major role in chronic heart failure in spite of the fact we have proposed sympathetic blockade as concept for regulating cardiac failure.