Thyroxine treatment of hypothyroidism
The treatment of hypothyroidism is thyroid replacement for life (see webpage on thyroid Function). Levothyroxine (T4) and triiodothyronine (T3) are the thyroid hormones that circulate throughout the bloodstream. The thyroid gland is the sole source of T4. However, only 10% to 20% of T3 is made in the thyroid gland; the remaining 80% to 90% is produced when T4 is broken down into T3 by other organs in the body.
The drug of choice is synthetic levothyroxine (T4), which is converted in the body to the active hormone T3. A single T4 tablet has a long life in the body and its effect lasts for several weeks (half-life seven days). This means that there is a lot of leeway should you inadvertently forget to take your dose on a particular day. This plus the fact that the conversion from T4 to T3 is constant and stable makes T4 an ideal drug in hypothyroidism.
Taking Thyroxine tablets
In the younger patient the treatment is simple, starting usually with 100ug T4 per day for women and 125ug per day for men, adjusting the dose until the TSH is normal. In the elderly or frail patient with a new diagnosis of hypothyroidism a much more cautious approach is used with careful monitoring of the heart, and replacement starting with doses as low as 25ug per day, slowly increasing the dose until the TSH is normal. Improvement is slow and may take many weeks. This of course does not apply when replacing thyroid function immediately after thyroidectomy, when full doses may be given from the beginning.
The tablets are best taken on an empty stomach before breakfast. Dose requirements will change with age, pregnancy and gross weight change. The absorption of thyroxine can be affected by other drugs being taken at the same time, which may require adjustments to be made:
- Calcium supplements, ferrous sulphate (iron supplements), multivitamins, antacids containing aluminium hydroxide, proton pump inhibitors like omeprazole, and cholestyramine
These drugs all reduce the absorption of thyroxine and should be taken at a different time of the day to thyroxine, usually 2-4 hours later, or at the other end of the day. However if the TSH remains normal while on both drugs then no change needs to be made. - Phenytoin, carbamazepine, phenobarbitone and rifampicin
These drugs increase the clearance of thyroxine from the body so that larger doses of thyroxine may be required, depending on the TSH level.
Any patient with permanent hypothyroidism (low thyroid state) from any cause will need thyroid hormone replacement for the rest of their lives. There are four available brands of synthetic levothyroxine in Australia is branded either Oroxine, Eutroxsig, Eltroxin and Levoxine. The substance in the pill is identical to the thyroid hormone (T4) produced by the thyroid. Most people will need about 100 to 125 micrograms a day to replace thyroid function completely, which amounts to one or two pills every day, for life.
At times it may be necessary to prescribe doses of thyroxine other than the standard available doses of 50, 100 or 200 micrograms (mcg). in order to achieve the best results. These intermediate doses can be achieved by taking different doses on different days of the week, such as taking (for example) 100mcg and 150mcg on alternate days, to average out at 125mcg per day. This averaging is possible because thyroxine stays in the blood for many days after you swallow the tablets, and the levels do not fluctuate much at all from day to day.
When a patient takes levothyroxine, the level of T4 in the blood rises and falls slowly. On the other hand, when a patient takes medications containing the much more potent T3, the blood level of T3 rises quickly to hyperthyroid levels and falls rapidly. Therefore, preparations containing T3 cause patients to become hyperthyroid several hours a day and are not recommended.
It is important to note:
- If the type, brand, or dosage of levothyroxine is changed, patients should have their blood levels of thyroid hormones checked two to three months later.
- Taking more levothyroxine than prescribed does not speed up recovery and may cause hyperthyroidism and osteoporosis (thinning of the bones).
- Taking the proper dose of levothyroxine will not increase a patient's risk of developing osteoporosis.
- Levothyroxine treatment is not indicated for patients with fatigue, obesity, or infertility unless the patients also have a confirmed diagnosis of hypothyroidism.
- The warning on some non-prescription cold and flu preparations to avoid taking them if the patient has thyroid disease does not apply to hypothyroid patients taking levothyroxine in the prescribed amounts.
- Pregnant women and nursing mothers can safely take levothyroxine. In fact, patients with inadequately treated hypothyroidism have an increased risk of miscarriage.
The problem of a persistently raised TSH despite an adequate replacement dose of thyroxine is common and is almost always due to haphazard taking of thyroxine. In such non-compliant patients the total weekly dose may be taken on a single day in the week. This regimen is safe and usually effective, but not ideal.
If on treatment the patient still does not feel well and the TSH is below 2.5mU/L then the diagnosis of hypothyroidism is almost certainly wrong and alternative diagnoses must be considered.
Subclinical hypothyroidism
If there is subclinical hypothyroidism, with the TSH just above the normal range and thyroxine levels within the reference range, the management is controversial. It is known that such patients with positive antibodies have a conversion rate to overt hypothyroidism of around 5% per year. Those patients with little or no symptoms can be safely watched with a yearly clinical review.
If the antibodies are negative the review can be done three yearly. When patients have symptoms consistent with hypothyroidism a therapeutic trial of thyroxine for 3-6 months is justified. If the symptoms vanish then the thyroxine should be continued, but if not it should be stopped. Women with subclinical hypothyroidism intending pregnancy should be treated.
In subclinical hypothyroidism it is best to start with the calculated full replacement dose, aiming at a TSH of 2.5 iU/l or lower. It must be remembered that that TSH may take 3 to 6 months to return to well within the reference range.