The influence of circulating thyroxine serum concentration on hepatic thyroxine deiodinating activity in rats

Should the dose of levothyroxine be changed in hypothyroidism patients fasting during Ramadan?

Background/aim

Muslims worship by fasting from predawn (suhoor) until sunset (iftar) for 30 days in the religious month of Ramadan. In addition to prolonged hunger, patients fasting with a diagnosis of hypothyroidism take their doses of levothyroxine (LT4) outside of daytime fasting hours. The purpose of our study is to compare the values of hypothyroid patients which have been obtained through thyroid function tests before and after Ramadan.

Materials and methods

Ninety-seven patients; ranging from 18 to 65 years old, who were followed with a diagnosis of hypothyroidism, who fasted during Ramadan, and who had no change of their LT4 dose for at least 6 months were included in the study.

Results

The median serum thyroid-stimulating hormone (TSH) level of patients prior to fasting was 2.19 mIU/L, while median serum TSH after fasting was 2.73 mIU/L. Serum TSH values after Ramadan increased significantly compared to those prior to Ramadan (P = 0.004).

Conclusion

Our study demonstrates a significant increase in serum TSH levels after Ramadan but no significant change in serum free thyroxine (fT4) levels in hypothyroidism patients who are fasting. It may be appropriate to take precautions by making a small increase in LT4 dose before Ramadan in some hypothyroid patients wishing to fast.

Retinal S-opsin dominance in Ansell's mole-rats (Fukomys anselli) is a consequence of naturally low serum thyroxine

Mammals usually possess a majority of medium-wavelength sensitive (M-) and a minority of short-wavelength sensitive (S-) opsins in the retina, enabling dichromatic vision. Unexpectedly, subterranean rodents from the genus Fukomys exhibit an S-opsin majority, which is exceptional among mammals, albeit with no apparent adaptive value. Because thyroid hormones (THs) are pivotal for M-opsin expression and metabolic rate regulation, we have, for the first time, manipulated TH levels in the Ansell's mole-rat (Fukomys anselli) using osmotic pumps. In Ansell's mole-rats, the TH thyroxine (T4) is naturally low, likely as an adaptation to the harsh subterranean ecological conditions by keeping resting metabolic rate (RMR) low. We measured gene expression levels in the eye, RMR, and body mass (BM) in TH-treated animals. T4 treatment increased both, S- and M-opsin expression, albeit M-opsin expression at a higher degree. However, this plasticity was only given in animals up to approximately 2.5 years. Mass-specific RMR was not affected following T4 treatment, although BM decreased. Furthermore, the T4 inactivation rate is naturally higher in F. anselli compared to laboratory rodents. This is the first experimental evidence that the S-opsin majority in Ansell's mole-rats is a side effect of low T4, which is downregulated to keep RMR low.

Thyroid diseases and Ramadan

In the month of Ramadan, patients with thyroid diseases, most of the time, do not need treatment adjustments and can fast safely without any health hazards. Patients with hypothyroidism taking thyroxine can take their tablets on an empty stomach at bedtime instead of half an hour before Sehr. Patients with hyperthyroidism, on methimazole/carbimazole can continue their dose in once or twice daily regimes, while those on propylthiouracil need to be switched. Hyperthyroid patients with severe symptoms should start treatment immediately and can avoid fast for few days after a consultation with their religious scholar.

Can Levothyroxine Be Taken as Evening Dose? Comparative Evaluation of Morning versus Evening Dose of Levothyroxine in Treatment of Hypothyroidism

152 drug naïve primary hypothyroid patients were divided into morning (Group 1) and evening (Group 2) dosing group and evaluated for change in biochemical profile, physical functioning and Quality of Life during the course of 12 weeks of study. At the end of 12 weeks 70 (90.90%) subjects in Group 1 and 72 (96%) in Group 2 achieved euthyroidism. On evaluation clinical symptoms and total clinical scores improved in both the groups at the end of 6 and 12 weeks. Significant improvement in thyroid profile was seen in both the groups at the end of 6 and 12 weeks (P value <.0001). On intergroup comparison, no significant difference in thyroid profile was seen at 6 and 12 weeks between the morning and the evening dose group. Similar dose of levothyroxine was required to achieve euthyroidism in both the groups. Though an early restoration of euthyroidism was seen in evening group, the difference when compared to the morning group was not statistically significant. On assessment of QoL, statistically significant improvement in various parameters was seen in both the groups. Hence, from the study we inferred that evening dose is as efficacious as morning dose and provides an alternate dosing regimen.

Effects of evening vs morning thyroxine ingestion on serum thyroid hormone profiles in hypothyroid patients

OBJECTIVE

Standard drug information resources recommend that l-thyroxine be taken half an hour before breakfast on an empty stomach, to prevent interference of its intestinal uptake by food or medication. We observed cases in which TSH levels improved markedly after changing the administration time of l-thyroxine to the late evening. We therefore conducted a pilot-study to investigate whether l-thyroxine administration at bedtime improves TSH and thyroid hormones, and whether the circadian rhythm of TSH remains intact. DESIGN Patients were studied on two occasions: on a stable regimen of morning thyroxine administration and two months after switching to night-time thyroxine using the same dose. On each occasion patients were admitted for 24 h and serial blood samples were obtained.

PATIENTS

We investigated 12 women treated with l-thyroxine because of primary hypothyroidism, who used no medication known to interfere with l-thyroxine uptake.

MEASUREMENTS

Patients were admitted to hospital and blood samples were obtained at hourly intervals for 24 h via an indwelling catheter. Following this first hospital admission, all women were asked to switch the administration time from morning to bedtime or vice versa. After 2 months they were readmitted for a 24-h period of hourly blood sampling. Blood samples were analysed for serum TSH (immunometric assay), FT4 and T3 (competitive immunoassay), T4 and rT3 (radioimmunoassay), serum TBG (immunometric assay) and total protein and albumin (colourimetric methods).

RESULTS

A significant difference in TSH and thyroid hormones was found after switching to bedtime administration of l-thyroxine. Twenty-four-hour average serum values amounted to (mean +/- SD, morning vs bedtime ingestion): TSH, 5.1 +/- 0.9 vs 1.2 +/- 0.3 mU/l (P < 0.01); FT4, 16.7 +/- 1.0 vs 19.3 +/- 0.7 pmol/l (P < 0.01); T3, 1.5 +/- 0.05 vs 1.6 +/- 0.1 nmol/l (P < 0.01). There was no significant change in T4, rT3, albumin and TBG serum levels, nor in the T3/rT3 ratio. The relative amplitude and time of the nocturnal TSH surge remained intact.

CONCLUSIONS

l-thyroxine taken at bedtime by patients with primary hypothyroidism is associated with higher thyroid hormone concentrations and lower TSH concentrations compared to the same l-thyroxine dose taken in the morning. At the same time, the circadian TSH rhythm stays intact. Our findings are best explained by a better gastrointestinal uptake of l-thyroxine during the night.

Thyroid status and hepatic thyroxine deiodinating activity under high-dose long-term nitrendipine

The influence of high doses of oral nitrendipine on the hypophyseal-thyroid axis and on peripheral thyroxine metabolism was studied in baboons. Administration of 320 mg oral nitrendipine per kg body weight (b.wt.) for three months caused a hypothyroid state with decreased values for thyroxine and reverse triiodothyronine, elevated TSH, but unchanged triiodothyronine; the lower doses investigated (24 and 48 mg/kg b.wt.) were without any effect. High doses of nitrendipine concomitantly increased hepatic 5'-deiodinating activity by a rise in Vmax, which could be attributed to an increase in the deiodinating enzyme content. Normal T3 serum levels in the presence of low T4 serum concentrations under high dose nitrendipine can be ascribed, at least in part, to the enhanced peripheral 5'-deiodination.