Assessment of thyroid function during pregnancy

Thyroxine secretion rates during pregnancy in the rat

Thyroxine secretion rates (TSR) at various stages of pregnancy in rats were measured by the radiothyroxine pool-turnover method. Groups of rats included normal controls (non-pregnant), days 5, 10, 20, and 22 of pregnancy and near term (within 24 hours prior to parturition if past 22 days of pregnancy). Each animal had blood samples taken just prior to injection of 10 microCi L-thyroxine (L-T4)-131I and at 12, 24, 36, and 48 hours afterwards. Determinations of thyroxine iodine (T4-I), volume of distribution (VD), fractional turnover rate (K), and L-T4 pool size were made from these samples. TSR was calculated as the product of (1.54) (T4-I) (VD) (K). An increase in TSR occurred near term, 3.65 micrograms L-T4/day compared to 1.23 micrograms L-T4/day in the controls, was primarily due to an increase in VD from 33.9 ml in the controls to 90.0 ml near term. A rapid increase in TSR to 3.65 micrograms L-T4/day just prior to term was postulated to be due to a stimulatory action of relaxin in the presence of estrogen upon L-T4 utilization. The mechanism of the increase in TSR in pregnant rats is probably due to the effect of several hormonal changes in maternal tissues throughout pregnancy and to adaptation of the pregnant animals to higher metabolic needs of the rapid developing fetuses.

Thyroid function tests in thyroid and nonthyroid disease

Modern day evaluation of thyroid disorders requires a combination of accurate clinical judgement and reliable, sensitive, and specific thyroid functions tests. Principle among the latter are thyroxine (T4) 3, 5, 3'-triiodothyronine (T3), and thyroid-stimulating hormone (TSH). Also playing an important role in special situations are free thyroxine, an assessment of bound and unbound thyroid-binding globulin, TRH stimulation, long-acting thyroid stimulator (LATS), antibodies to thyroid hormone and to thyroid receptors. Basic to interpretation of these tests in the clinical setting is a comprehension of the relationship of the hypothalamus, the pituitary, and the thyroid gland as well as a knowledge of the peripheral metabolism of thyroxine and triiodothyronine. The role of each of these laboratory tests in the evaluation of hyper- and hypometabolic states, their alteration in nonthyroid and other endocrine disorders, and the effects of environmental and physiological factors on these tests are reviewed.

Acute hyperthyroidism during pregnancy: a case report and critical analysis

A rare case of acute hyperthyroidism during pregnancy is described and the management is discussed. Prompt subtotal thyroidectomy may be superior to the use of antithyroid drugs in particular circumstances. Fetal outcome of hyperthyroid mothers is discussed in the light of recent long-term follow-up studies and immunogenetic data. Women considered to be hyperthyroid are at high-risk for obstetrical and neonatal problems: early diagnosis of this condition is of paramount importance.

Effects of heroin addiction on thyrotrophin, thyroid hormones and porlactin secretion in men

Pituitary-thyroid function in male heroin addicts and addicts after abstinence (ex-addicts) was studied and compared with that of healthy euthyroid men. In heroin addicts the increases in circulating total thyroxine and triiodothyronine levels were accompanied by an increase in the thyroid hormone uptake test. These changes may reflect a quantitative increase in thyroxine binding globulin. Reverse triiodothyronine concentrations in heroin addicts were normal. The thyrotrophin-releasing hormone elicited a diminished thyrotrophin response in heroin addicts which was significantly different from that in control subjects and ex-addicts. An elevation of serum prolactin was noted in heroin addicts, while ex-addicts had normal levels. Gradual recovery of pituitary-thyroid function occurred after heroin withdrawal.

The physiology of thyroid function in pregnancy

Pregnancy has a variety of effects on maternal thyroid function. Thyroid gland enlargement is common particularly in areas of relative iodine deficiency. The renal clearance of iodine is increased in pregnancy and together with an increased volume of iodine distribution, leads to a low plasma inorganic iodine and thus increases the thyroidal iodine clearance. However, the absolute iodine uptake and hormone production rate remain unchanged. There is an increase in the serum thyroxine (T4)and triiodothyronine (T3) concentration largely due to an increase in thyroid hormone-binding proteins. Free thyroxine and free T3 remain unchanged in pregnancy as does the Free Thyroxine Index, which gives the single most accurate measure of thyroid function. The placenta secretes a number of thyroid stimulators including human chorionic gonadotrophin and possibly chorionic thyrotrophin and molar thyrotrophin whose physiological role is to date poorly understood. The fetal thyroid develops independently, and although fetal T4 CONCENTRATION RISES PROGRESSIVELY To maternal by term, the T3 concentration is markedly reduced owing to preferential formation of inactive reverse T3.

Hypothyroidism in pregnancy

Two mothers with hypothyroidism gave birth to normal infants. In one patient there was good evidence that maternal hypothyroidism improved during the last trimester due to placental transfer of fetal thyroid hormone. If pregnancy is advanced when the diagnosis of hypothyroidism is made, it would seem that replacement therapy may be withheld without detriment to the fetus.

Altered patterns of thyroid hormones in serum and urine in pregnancy and during oral contraceptive therapy

Serum total, percentage free fraction and absolute serum free hormone concentration of thyroxine and triiodothyronine were measured in control, pregnant and oral contraceptive users, together with the daily urinary losses of unconjugated thyroid hormones. Increased urinary losses of both hormones, in particular thyroxine, were apparent in pregnancy and these could not be explained in terms either of an increased filtered load of hormone or the presence of proteinuria. The possible existence of filterable small-molecular weight hormone-binding substances in the urine of pregnant patients is discussed. It is concluded that assay of urinary thyroid hormones during pregnancy is of limited diagnostic value because of overlap with thyrotoxic values.